Matter of preference. I played with CM7 for about an hour, didn't like the skin of the dialer, sms app and few other things, but is real fast and you theme it almost at will. Went with MIUI, quite fast too, but it has a much cleaner look overall, very similar to iOS without the clunky app drawer which became a joke to me with Gingerbread, not that many widgets but that's ok with me, I barely use them anyways. Updates quite often, overall UI design is much better, but if don't like iOS might be an issue.
Quite pleased with MIUI, once again, my personal opinion.
I love cm7 and is my rom of choice. Like stated above though, it all depends on your preference. I like miui as well and you cant go wrong with either. I personally like using widgets and have seven screens. The speed is great and you can theme alot, especially if running a launcher. Good luck.