Compare two Excel sheets by Cell Content
This answer assumes that all Names in the data are unique. There is no
  provision in this example to handle duplicate Names except to issue a
  note in the debug output.


This answer will involve Dictionaries. Please review this website for complete information on how and why they are an efficient way to store unique data. The short answer is that you can create a large dictionary by looking at a unique "key", which is a string that uniquely represents some data that you want to track. In your case, you've asserted that all of the Names are unique. Dictionaries exist for speedy access to any single entry -- no looping through 200k entries to find the one you want. Use your unique key string and you have near-instant access to the data associated with that key.

For your situation, my example builds two Dictionaries, one for each set of data. The keys are the Names. The values associated with each key (Name) is the row number on which each Name is used. The row numbers will play a major role later on.

As with the other Code Review for your previous version of code, I'll reiterate:


Identify your function parameters ByRef or ByVal
Declare your variables as close to their first use as possible.


As an example:

Dim arr1 As Variant
Dim arr2 As Variant
arr1 = BuildDataArrays(ws1, startRow:=2)
arr2 = BuildDataArrays(ws2, startRow:=2)


You'll see that there is a call to a BuildDataArrays function. That brings me to 


Functional Isolation. When your routine starts getting very long, that is the perfect time to begin breaking parts of the logic out into separate functions/subs. This is especially useful if you have repetitive logic where only the variable is different. This is the case for BuildDataArrays.


Give this function a worksheet and optionally the starting row or column and it determines the range of available data, returning a memory-based array. Breaking out code into separate routines is very helpful because it makes your main logic easier to follow.

Private Function BuildDataArrays(ByRef ws As Worksheet, _
                                 Optional ByVal startRow As Long = 1, _
                                 Optional ByVal startCol As Long = 1) As Variant
    '--- assumes all rows and columns are valid data
    Dim lastRow As Long
    Dim lastCol As Long
    Dim dataArea As Range
    Dim data As Variant
    With ws
        lastRow = .Cells(.Rows.Count, startRow).End(xlUp).row
        lastCol = .Cells(startCol, .Columns.Count).End(xlToLeft).Column
        Set dataArea = .Cells(startRow, startCol).Resize(lastRow - startRow + 1, _
                                                         lastCol - startCol + 1)
        data = dataArea
    End With
    BuildDataArrays = data
End Function



Use Dictionaries to collect your data. As with the previous point, this is a perfect opportunity to isolate the logic in a separate function.


The BuildDataDictionary function will accept your memory-based array and use the selected column of data as a unique key (currently defaulted to column "I").

Private Function BuildDataDictionary(ByRef data As Variant, _
                                     Optional ByVal keyColumn As Long = 8) As Dictionary
    Dim row As Long
    Dim name As String
    Dim names As Dictionary
    Set names = New Dictionary
    For row = LBound(data, 1) To UBound(data, 1)
        name = Trim$(data(row, keyColumn))
        If Len(name) &gt; 0 Then
            If Not names.Exists(name) Then
                '--- add the new name to the dictionary and save the row number
                names.Add name, row
            Else
                '--- if you get here, it means that the Name is NOT unique
                '    and you'll have to change your logic, or change the name
                Debug.Print "ERROR: Duplicate name detected on " &amp; _
                            " on row " &amp; row &amp; ": '" &amp; name &amp; "'"
            End If
        End If
    Next row
    Set BuildDataDictionary = names
End Function


Next we'll build the resulting report data. According to your description, the report will consist of all data rows (each with a unique Name), with any differences noted in the data itself. In your original post, you are assuming that the larger of the two row counts for the sheets will be your output array. This isn't true.

Consider that, by definition, all of the data from Sheet1 is unique (because each row's Name is unique). That means if you have 10 rows of data on Sheet1, your output data will have at least ten rows. It's possible that your data on Sheet2 also has ten rows of data, and only one of those rows repeats a Name on Sheet1. So your resulting report of data will have 19 rows.

Dim totalRows As Long
totalRows = ws1Names.Count

'--- now add on the number of unique rows from the other sheet
Dim name As Variant
For Each name In ws2Names
    If Not ws1Names.Exists(name) Then
        '--- name is unique
        totalRows = totalRows + 1
    Else
        '--- name is not unique
    End If
Next name
Debug.Print "There are " &amp; totalRows &amp; " unique Names between the sheets"

'--- now build a correctly sized output array
'    ASSUMES both arrays have the same number of columns!!
Dim reportData As Variant
ReDim reportData(1 To totalRows, 1 To UBound(arr1, 2))


Now that we're about to generate the report data, we have to consider how to make note of any errors encountered. For this I'm using a Collection, which is a simple way to generate a running list of items. In this case, for each difference I'm adding a string that notes the row and column of each difference in the data arrays. I can use this later on to highlight the difference cells.

'--- and create an object to list which cells are different
Dim diffCells As Collection
Set diffCells = New Collection


After that, we simply move the data over to the report array, making note of any differences.

'--- we know that all Names are unique in sheet1, so move the all that
'    data from sheet1 into the report array
Dim row As Long
Dim col As Long
Dim ws1row As Long
Dim ws2row As Long
row = 1
For Each name In ws1Names
    If ws2Names.Exists(name) Then
        '--- this row will have a difference because the Names match!
        '    so get the rows for each sheet that match the name
        ws1row = ws1Names(name)
        ws2row = ws2Names(name)
        For col = 1 To UBound(reportData, 2)
            If arr1(ws1row, col) = arr2(ws2row, col) Then
                reportData(row, col) = arr1(ws1row, col)
            Else
                '--- note the different values in the cell and add the
                '    row and column to the difference list
                reportData(row, col) = arr1(ws1row, col) &amp; " &lt;&gt; " &amp; _
                                       arr2(ws2row, col)
                diffCells.Add CLng(row) &amp; "," &amp; CLng(col)
            End If
        Next col
    Else
        '--- this is a unique row, so a straight copy of all columns
        For col = 1 To UBound(reportData, 2)
            reportData(row, col) = arr1(row, col)
        Next col
    End If
    row = row + 1
Next name

'--- the remaining data are the unique rows that exist in sheet2
'    the "row" variable count is continued in this loop
For Each name In ws2Names
    If Not ws1Names.Exists(name) Then
        '--- this is a unique row, so a straight copy of all columns
        ws2row = ws2Names(name)
        For col = 1 To UBound(reportData, 2)
            reportData(row, col) = arr2(ws2row, col)
        Next col
        row = row + 1
    End If
Next name


The final step is to output the report data. In my example, I am not creating a new workbook, but only creating a new worksheet. You can un-comment some code lines there to change it back for your purposes.

If diffCells.Count &gt; 0 Then
    Dim report As Workbook
    Dim reportWS As Worksheet
    'Set report = Workbooks.Add           'un-comment to report to a new workbook
    'Set reportWS = report.ActiveSheet    'un-comment to report to a new workbook
    Set reportWS = ThisWorkbook.Sheets.Add   'comment to report to a new workbook

    '--- copy the resulting report to the worksheet
    Dim reportArea As Range
    Set reportArea = reportWS.Range("A1").Resize(UBound(reportData, 1), UBound(reportData, 2))
    With reportArea
        .Value = reportData
        .Columns("A:B").ColumnWidth = 25

        '--- now highlight the cells that are different
        Dim rowcol As Variant
        Dim parts() As String
        For Each rowcol In diffCells
            parts = Split(rowcol, ",")
            With .Cells(CLng(parts(0)), CLng(parts(1)))
                .Font.Bold = True
                .Font.ColorIndex = 3
            End With
        Next rowcol
    End With
    Debug.Print "Report Generated secs " &amp; Timer - tm
End If


Here is the whole code module in one block:

Option Explicit

Sub test1()
    Compare2WorkSheets Sheet1, Sheet2
End Sub

Sub Compare2WorkSheets(ByRef ws1 As Worksheet, ByRef ws2 As Worksheet)
    Dim tm As Double
    tm = Timer

    'Application.ScreenUpdating = False
    'Application.Calculation = xlCalculationManual
    'Application.EnableEvents = False

    '--- establish the data in the arrays, skip the header row
    Dim arr1 As Variant
    Dim arr2 As Variant
    arr1 = BuildDataArrays(ws1, startRow:=2)
    arr2 = BuildDataArrays(ws2, startRow:=2)

    '--- buidl a dictionary of Names for each worksheet
    Dim ws1Names As Dictionary
    Dim ws2Names As Dictionary
    Set ws1Names = BuildDataDictionary(arr1)
    Set ws2Names = BuildDataDictionary(arr2)

    '--- we don't know how many rows the report will be, so compare
    '    names between the two sheets to find out. it's basically
    '    the sum of the number of unique names between the sheets
    Dim totalRows As Long
    totalRows = ws1Names.Count

    '--- now add on the number of unique rows from the other sheet
    Dim name As Variant
    For Each name In ws2Names
        If Not ws1Names.Exists(name) Then
            '--- name is unique
            totalRows = totalRows + 1
        Else
            '--- name is not unique
        End If
    Next name
    Debug.Print "There are " &amp; totalRows &amp; " unique Names between the sheets"

    '--- now build a correctly sized output array
    '    ASSUMES both arrays have the same number of columns!!
    Dim reportData As Variant
    ReDim reportData(1 To totalRows, 1 To UBound(arr1, 2))

    '--- and create an object to list which cells are different
    Dim diffCells As Collection
    Set diffCells = New Collection

    '--- we know that all Names are unique in sheet1, so move the all that
    '    data from sheet1 into the report array
    Dim row As Long
    Dim col As Long
    Dim ws1row As Long
    Dim ws2row As Long
    row = 1
    For Each name In ws1Names
        If ws2Names.Exists(name) Then
            '--- this row will have a difference because the Names match!
            '    so get the rows for each sheet that match the name
            ws1row = ws1Names(name)
            ws2row = ws2Names(name)
            For col = 1 To UBound(reportData, 2)
                If arr1(ws1row, col) = arr2(ws2row, col) Then
                    reportData(row, col) = arr1(ws1row, col)
                Else
                    '--- note the different values in the cell and add the
                    '    row and column to the difference list
                    reportData(row, col) = arr1(ws1row, col) &amp; " &lt;&gt; " &amp; _
                                           arr2(ws2row, col)
                    diffCells.Add CLng(row) &amp; "," &amp; CLng(col)
                End If
            Next col
        Else
            '--- this is a unique row, so a straight copy of all columns
            For col = 1 To UBound(reportData, 2)
                reportData(row, col) = arr1(row, col)
            Next col
        End If
        row = row + 1
    Next name

    '--- the remaining data are the unique rows that exist in sheet2
    '    the "row" variable count is continued in this loop
    For Each name In ws2Names
        If Not ws1Names.Exists(name) Then
            '--- this is a unique row, so a straight copy of all columns
            ws2row = ws2Names(name)
            For col = 1 To UBound(reportData, 2)
                reportData(row, col) = arr2(ws2row, col)
            Next col
            row = row + 1
        End If
    Next name

    Debug.Print " Calc secs " &amp; Timer - tm
    If diffCells.Count &gt; 0 Then
        Dim report As Workbook
        Dim reportWS As Worksheet
        'Set report = Workbooks.Add           'un-comment to report to a new workbook
        'Set reportWS = report.ActiveSheet    'un-comment to report to a new workbook
        Set reportWS = ThisWorkbook.Sheets.Add   'comment to report to a new workbook

        '--- copy the resulting report to the worksheet
        Dim reportArea As Range
        Set reportArea = reportWS.Range("A1").Resize(UBound(reportData, 1), UBound(reportData, 2))
        With reportArea
            .Value = reportData
            .Columns("A:B").ColumnWidth = 25

            '--- now highlight the cells that are different
            Dim rowcol As Variant
            Dim parts() As String
            For Each rowcol In diffCells
                parts = Split(rowcol, ",")
                With .Cells(CLng(parts(0)), CLng(parts(1)))
                    .Font.Bold = True
                    .Font.ColorIndex = 3
                End With
            Next rowcol
        End With
        Debug.Print "Report Generated secs " &amp; Timer - tm
    End If
    'Application.ScreenUpdating = True
    'Application.Calculation = xlCalculationAutomatic
    'Application.EnableEvents = True

    If diffCells.Count &gt; 0 Then
        Debug.Print diffCells.Count &amp; " cells contain different data!"
    Else
        Debug.Print "No differences found between the sheets."
    End If
End Sub

Private Function BuildDataArrays(ByRef ws As Worksheet, _
                                 Optional ByVal startRow As Long = 1, _
                                 Optional ByVal startCol As Long = 1) As Variant
    '--- assumes all rows and columns are valid data
    Dim lastRow As Long
    Dim lastCol As Long
    Dim dataArea As Range
    Dim data As Variant
    With ws
        lastRow = .Cells(.Rows.Count, startRow).End(xlUp).row
        lastCol = .Cells(startCol, .Columns.Count).End(xlToLeft).Column
        Set dataArea = .Cells(startRow, startCol).Resize(lastRow - startRow + 1, _
                                                         lastCol - startCol + 1)
        data = dataArea
    End With
    BuildDataArrays = data
End Function

Private Function BuildDataDictionary(ByRef data As Variant, _
                                     Optional ByVal keyColumn As Long = 8) As Dictionary
    Dim row As Long
    Dim name As String
    Dim names As Dictionary
    Set names = New Dictionary
    For row = LBound(data, 1) To UBound(data, 1)
        name = Trim$(data(row, keyColumn))
        If Len(name) &gt; 0 Then
            If Not names.Exists(name) Then
                '--- add the new name to the dictionary and save the row number
                names.Add name, row
            Else
                '--- if you get here, it means that the Name is NOT unique
                '    and you'll have to change your logic, or change the name
                Debug.Print "ERROR: Duplicate name detected on " &amp; _
                            " on row " &amp; row &amp; ": '" &amp; name &amp; "'"
            End If
        End If
    Next row
    Set BuildDataDictionary = names
End Function



  EDIT: added an example on how to call the routine from a button click


It seems that you're adding an ActiveX command button to your worksheet. In this case, the CommandButton1_Click() method will be executed in the Sheet1 module. Take the code above with the Compare2WorkSheets routine and paste it into a regular code module. Then, in your sheet1 module, fix up your button-click code like this:

Option Explicit

Private Sub CommandButton1_Click()
    Dim myWorkbook1 As Workbook
    Dim myWorkbook2 As Workbook

    '--- if Sheet1 is contained in the workbook where the code is running, use this
    Set myWorkbook1 = ThisWorkbook

    '--- if Sheet1 is in a different -- but already open -- workbook, use this
    Set myWorkbook1 = Workbooks("the-already-open-workbook-filename.xlsx")

    '--- if Sheet1 is in a different -- but unopened -- workbook, use this
    Set myWorkbook1 = Workbooks.Open("the-workbook-filename-to-open.xlsx")

    '--- you can make the same decisions for setting myWorkbook2
    Set myWorkbook2 = Workbooks.Open("C:\Temp\testreport1.xlsx")


    Compare2WorkSheets myWorkbook1.Worksheets("Sheet1"), myWorkbook2.Worksheets("Sheet1")

    myWorkbook1.Close
    myWorkbook2.Close
End Sub