What impact does UseMicrosoftUpdate have on the UpdateSource option when installing SQL Server 2014? The ConfigurationFile.ini generated for SQL Server 2014 contains a new parameter that I cannot find documented anywhere - UseMicrosoftUpdate.  

What impact does this parameter have on the UpdateSource and UpdateEnabled parameters?
UseMicrosoftUpdate - Setting it to TRUE - it will change the computer settings to include the latest updates when you scan for Windows Update.
I would suggest to set it FALSE and you decide what updates you want for SQL Server in-terms of SPs/CUs.
From BOL :

UpdateEnabled - Specify whether SQL Server setup should discover and include product updates. The valid values are True and False or 1 and 0. By default, SQL Server setup will include updates that are found.
UpdateSource - Specify the location where SQL Server setup will obtain product updates. The valid values are &quot;MU&quot; to search Microsoft Update, a valid folder path, a relative path such as .\MyUpdates or a UNC share. By default, SQL Server setup will search Microsoft Update or a Windows Update Service through the Windows Server Update Services

So the UseMicrosoftUpdate is more of a global thing governed as part of windows update vs UpdateEnabled is governed only during sql server installation/setup and UpdateSource is the location that will have sql server update files. Thats my understanding.
Honestly, I will not even enable any of those, since any SP/CU has to be regression tested and lately there are many problems with SP/CUs that can screw up many things. They are addressed with subsequent hotfixes.
With introduction of SQL Server 2016, Microsoft has made changes to their Incremental Servicing Model (ISM)

As of January CU releases, we now recommend ongoing, proactive installation of CU’s as they become available. You should plan to install a CU with the same level of confidence you plan to install SPs (Service Packs) as they are released. This is because CU’s are certified and tested to the level of SP’s.