How does performance of SQL Server CE and SQL Server Express compare?
For your case and reasons to upgrade

merge replication is available in SQL Server CE 3.5

Stored procedures, views, triggers is not available in SQL Server CE 3.5

for a basic comparison between the 2 editions in your case working on a net book

1.
SQL Server CE 3.5 can Runs as in-process with application
SQL Server Express runs as a service

2.
SQL CE supports maximum size of 4GB
SQL Express can go up to 10GB

3.
SQL CE Doesn't Support FILESTREAM
SQL Express Supports FILESTREAM


Procedural T-SQL - Select Case, If, features is not supported in SQL CE
Procedural T-SQL - Select Case, If, features is supported in SQL Express


5.
Distributed transactions is not supported in SQL CE but supported in SQL express


Native XML, XQuery/XPath is not supported in SQL CE but supported in SQL express


7.
Stored procedures, views, triggers is not supported in SQL CE but supported in SQL express


Role-based security is not supported in SQL CE but supported in SQL express


9.
concurrent connections supported in SQL CE is limited to 100 but unlimited in sql express

this is a useful blog focused on sql CE 

for the comparison between the 2 in your specific environment you can use these tools

SQL LOAD TEST
SQL STRESS TEST TOOL

and check this post on SO about sql statment performance
https://stackoverflow.com/questions/1555471/what-tools-are-available-to-test-sql-statement-performance