Sense 2.1 vs 2.5: until recently, the advantage of 2.1 was that it could do landscape, whereas 2.5 couldn't. Now, 2.5 can do landscape, so in my opinion, there's no sense (lame pun intended) in getting 2.1.
As for which ROM, well, that's up to you. I haven't tried them all, but I like
RootRom.