There are pros and cons to both AOSP ROM's and Sense ones.
Sense is prettier and adds some nice touches, but is slow and feels cumbersome.
AOSP (CM7/MIUI etc) are relatively bare, meaning they're wide open for customisation and on the flip side are much slicker than sense based ROMs.
IMO, CM7 is the way to go. It can take a little getting used to, but once you do, you won't look back!