There's a feature matrix here, although you may have already seen it:
http://msdn.microsoft.com/en-us/library/cc645993(v=sql.105).aspx
Don't trust software resellers, I've bought from them more than once and had to spend literally hours arguing over licensing rules with them. It's in their interest to sell you the most expensive package.
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Note to developers:Want to get the best help? Click here https://www.sqlservercentral.com/articles/forum-etiquette-how-to-post-datacode-on-a-forum-to-get-the-best-help (Jeff Moden)
My blog: http://uksqldba.blogspot.com
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