I currently own a HK USPf .45. That said, both are EXCELLENT pistols and often compared to each other. The USP is my only non-1911 .45, which I am really happy with.
So, there are plenty of advantages and disadvantages to both platforms. We'll start with the USP. With the USP, it is double stack, so you'll have the additional rounds (or more if you had standard cap mags already). The gun is available in a variety of modes. You can carry it cocked and locked, or DA/SA. Heck, you can even carry it DA/SA locked if you wanted to. Only thing is, us high thumb shooters riding the safety lever risks the chance of decocking the gun halfway through a string. The trigger is horrible in DA, and heavy in SA. It'll also have a long reset. You can get a match trigger installed, but it won't be a huge improvement, unless you take it to a gunsmith, or the guys at Grayguns. P.S. The stock sights REALLY suck. Similar to the Glock sights, but with those huge white dots that look like eyeballs staring back at you. Swap em' out for Heinie Straight Eights for a beautifully crisp sight picture.
The P220 on the other hand, is a different platform. The Classic P220 comes in DA/SA, which usually has a better trigger. It's definitely lighter, with the DA trigger pull a slight tad shorter than the USP. The P220 comes with better sights (IMO) than the USP, which I definitely prefer the bar-dot sights than the 3 dots on the USP. There are also some beautiful P220s, as you can swap out grips to your hearts content, such as nills, or the Equinox grips. The only gripe (a good one too) are the grip screws. They shoot loose on a regular basis, so you'll have to keep an eye on them, or blue loctite them. Bruce and Scott @ GGI can make a really good gun quite incredible. But as a stock pistol, the trigger is going to be "good",but it'll also have a long trigger reset just like the USP. Also, if you shoot high thumb, your thumb might ride on the slidestop and prevent slidelock on the last round.
I should close stating that while the P220 is an excellent platform, I personally don't have a use for one. I got the USP because I felt it was more durable. The P220 can hold it's own also, but I wanted something different other than 1911s and durability was a major factor for me. The USP won that one.