I have owned the HK45 and HK45 Tactical. They are now sold and I still have my USP 45.... and I'm wanting to pick up a USP 45 compact.
I'm kind of a big USP fanboy. Of course, I'm also a big P2000 and P30 fanboy, too... The HK45 just doesn't work well for me for some reason.
Like others have said, it's really a personal decision. You'll ant to get both in your hands - or ideally, shoot them both - to decide which works better for you. The HK45 feels decent enough in my hand, I just found the grip to be too long for me(it's longer than the USP 45 while holding fewer rounds). The trigger on the USP is also better, in my opinion - especially with the match trigger kit.
The HK45 is a little faster cycling which means it returns to target a little sooner, but it also has more perceived recoil, in my opinion. I just like the USP.
