Well, I have never shot a SIg, but I have had other DA/SA guns. I do have a compact USP 9mm, and I had a fullsize USP 9mm up until last week. I bought a new gun, and had to pay for it somehow, so I sold the fullsize USP (I like the compact better, because the grip is thinner, so I kept that one).
It was a little bit too big for my hands. But, I also find the Sig to be a bit too big. Both are good guns, and if they fit your hand, either would be a good choice.
One thing I like about the USP is the frame mounted safety - that's why I bought them. I like being able to keep the gun cocked and locked (gun cocked w/ the safety on). Only frame mounted safeties really work well for this. The Sig will not be able to do this.
Both guns are both pretty expensive, and the mags are about the same cost (the Sig mags may be a bit more expensive, not sure).
U'll have to see which fits your hand the best - I have Walthers and that HK compact - I LOVE the mag release underneath the trigger guard. I find it so much easier than the standard button release (only gun I have w/ that kinda mag release is my Glock 34). Some people have a hard time transitioning to this type of mag release, but once ya do - it is SO much easier.