No question about it, the SIG 220. then again, I'm kind of biased as I have a Browning marked one. I have shot the H&K USP a fair bit and it is a good gun also. This is kind of comparing apples to oranges as the HK was designed as a special purpose weapons platform (if I have my models straight, I am not an HK expert) while the SIG was designed as a convenient to carry side arm. If I were forced to carry a handgun into a gunfight, the HK would be a serious contender for my choice otherwise, I would rather carry the 220 on my hip in the unlikely event of getting into a problem. So, pound for pound, give me the SIG as I would probably have it at hand if needed. Otherwise, the HK would be the choice.
As for quality and reliability, they are so close together that it comes down to the individual gun and the person maintaining it. These are top end guns with enough time (especially the 220) to have ironed out the consistant bugs. i'd say you both owe each other a bottle of your favored beverage. Come to think of it, that is a subject worth arguing about.