Both Sig and H&K put out some fine products. I'm most familiar with the .45's, but their products are fairly similar across the calibers. I can shoot them both with about the same degree of accuracy, although it took more rounds to master the USP than the P220. The trigger on most Sigs will have very little take-up, and is easy to master. The USP's trigger is longer and heavier in DA, and has more take-up in SA, but once you learn to stage the trigger a bit, you won't have any problems. Most people that fuss about the USP's trigger either don't know how to stage the trigger or are used to shooting 1911's. The Sig is a little more comfy for carry. It's smaller, and the decocker and slide release don't stick out like on the USP, although some would say that this fact is as much a liability as an asset. If you're a lefty it doesn't really matter. I liked the USP for its versatility. It has to be one of the most versatile handguns on the market. It can be suited to match just about anyone's preference, and I like C&L, which V1 does nicely. The H&K has more capacity, albeit in a bigger package.
Toss a coin, you're a winner either way IMO.
Jason