USP Compact is basically identical to the USP except a shorter barrel. The extra weight on the full size reduces recoil a little bit, but it's hardly noticeable. I kinda like the full size "just because", but my groupings are for the most part the same as with the compact.
What I love most about the USP series is the ergonomics. It just fits nicely in my hand, and I can operate everything with my thumb without having to put any major effort into it. I haven't had a chance to handle a P2000/3000, but I definately look forward to trying them out.