I own both, and I would offer these tid-bits of advice:
1. The USP .40 is definitely smaller in the grip area. If you have smaller hands, the .40 is the way to go.
2. Having said that, the HK full-size .45 is the single best .45 I have ever shot in my life. Supremely accurate, with LESS recoil than the .40. Just a great, great, handgun in every respect.
3. The .40 will cost about $100 less, if money is a factor, and ammo is a little cheaper. Magazines are a little less expensive for the .40 as well.
4. That's about it. The real big issue, aside from caliber, is the grip size. If your hands are okay with the .45, THEN GET IT. It is a joy to shoot/own/love/respect.