Honestly, HK makes probably the best firearms in the world and the USP is the premier pistol. I owned the .40 USP and I traded it, for an M-11. Why, well I just don't like the .40 caliber. I don't know why, i'm just used to the 9MM. If I wasn't interested in the .40 I wouldn't have bought it, but I just didn't like it.
The gun is the balls though. I'll get a 9MM in a heartbeat. Go for the .40 if that's what you shoot. It's all about personal preference anyway.