I have owned both and sold both.
The USP feels like a 2X4 in my hands. It has an avg DA trigger and a solid SA trigger but it is nowhere near as nice as a 1911 or BHP trigger. The DA is not as good as a Sig IMHO.
The HK45 felt better in the hand but has an avg DA trigger and a solid SA trigger but it is nowhere near as nice as a 1911 or BHP trigger. The DA is not as good as a Sig IMHO.
For me personally both guns where tweeners. I really have no need to carry a combat sized hi capacity 45. I do not need the extra capacity over a 1911 or Sig P220 because inside the home both are backed up by a shot gun.
To me you are asking a question we really cannot answer because IMHO its all about the ergos and those are subjective. It should all come down to which one feels better in your hand and which one you shoot better.
In the end if you like HKs you cannot go wrong with either one.