When I bought my server a couple of years back, it came with a pair of E5-2660s. I bought a pair of E5-2670s for about $125 and installed them instead. It was a frivolous waste, honestly, but the cost was low enough that I couldn't talk myself out of it (especially when I then sold the old pair of 2660s for about $65). But there's really no way to say what's best for you without, as
@LTCM said, knowing more about your use case.