I don't own either, but I have spent time with each pistol. It has been a while since I shot the USP, whereas I just had my first encounter with the 220 Wednesday night. I compared it with my Colt .45, and except for the fact that it is double action, I like the SIG alot.
The USP felt like it had a better double action trigger, but single action is what's important. The SIG wins there; it's SA pull is excellent. More slack than on my Colt, but just as crisp and a little lighter after the take-up. I managed excellent accuracy offhand. As for ergonomics, I believe it is the second most comfortable .45, after the 1911. The grip just feels wonderful, and the angle is perfect. I prefer the USP's ability to be carried cocked and locked, and still have a decocker as well. With the SIG you only get the decocker, no safety. I can live with it, but I really like C&L better than a long DA pull on the first shot.
Both are very well put together. I have heard better things about the HK's long-term reliability, but you pay more for it so that's what you should expect. You also get three more rounds with the USP. Oh, one more thing - the 220's mags are a real bitch to load!
Which one would I buy? Probably the SIG - it's cheaper and has a better grip. But I would eventually buy the USP, too!