I'm a big big fan of the H&K USP Compact. It's an awesome weapon.
I can't say how it stacks up against the SIG though since I've only shot a Sig once 15 years ago.
Both the SIG and the HK are excellent products. Both are well made, very reliable and very accurate (at least the full size HK USP is).
The P220 is a single stack gun but still has a relatively fat grip. It feels good if you don't have small hands.
The only criticism I have of the P220 is that it has relatively small sights compared to contemporary semi autos, the P220 is a very old design. I haven't examined any recent ones so they might have better sights now. You see a lot of SIG's on television, probably the P226 (9mm) models. Externally the P226 and the P220 are very similar and from a short distance away would be hard to tell apart.
There are a lot of good brands worthy of consideration besides HK and SIG- S&W, Beretta, Glock, Springfield, Kimber, Colt, Ruger, even Taurus. These cover a variety of price ranges and a lot of different action types. The decision of which to buy should consider the action type, price, size and caliber, as well as brand. A person would certainly be well served by any of these, but just as there are people that like axis lock knives or framelocks or whatever, there will be similar differences in these brands and models.