I have the USP comact in .45. I would have been just as happy with it in 9mm. In fact I was going to buy another 9mm for my CCW gun. There are a lot of posts about 45 vs 9mm , but it really comes down to shot placement IMHO.
If you shoot 9mm you will be able to practice much more,since I find that .45 ammo costs about 40% more than 9mm.