I am seeing two type of downloads:
SQL Server 2008 R2 Express, and
SQL Server 2008 R2 RTM Express.
What is the difference between the two?
I don't know specifically on those mssql versions, but generally RTM is an abbreviation for 'released to manufacturing' and is kind of the last beta version of a software application prior to the official release.
According to me, there is no special difference between them. Both are a very reliable, easy, powerful data management system.both of can delivers a rich set of features, data protection, applications info and and local data stores etc.
It's my own perception!