If you are building a project with .NET 4.0.2 or greater and want embedded database support, consider SQL Server Express LocalDB.
It's a relatively new addition to the Express family that features a smaller installation footprint and reduced management overhead (when compared to other editions of Express), yet it maintains the programmability aspects of SQL Server. That is, unlike Compact Edition, LocalDB does not require the installation of a separate ADO.NET provider to communicate with SQL.
See the following for more details:
SQL Express v LocalDB v SQL Compact Edition (MSDN Blogs)
SQL LocalDB vs SQL Server CE (Stack Overflow)
