Here's a grid showing the differences... well, some of them:
http://www.fmsinc.com/MicrosoftAccess/SQLServerUpsizing/express/index.html
So it kind of depends on what you need SQL Server to do. The free versions are okay if you're just serving data - so no SSRS, SSIS, SSAS etc. No SQL Server Agent (that the scheduler uses for running backups on a schedule etc), but you can do that with the OS.