Both will rust, both are reliable, both are accurate.
The SIG rusts more easily, but this is not an issue with a home defense gun unless you make it an issue by not maintaining the gun. Both are about equally reliable for me, no stoppages with factory ammo, not one, not of any kind.
Accuracy--here's where the paths diverge. The SIG is so accurate in my hands that this one virtue overshadows most others. I'm simply not a great shot with a handgun yet, but I have no problem dumping nine rounds into 2-3" from 15 yards with the 220. I can't do it with the USP, thought I love the thing.
OTOH, the light would be easier on the USP and replacing the gigantic sights would do a lot for practical accuracy, I suspect. But if the SIG fits your hand better as well, you have no choice here. The SIG will fit your hand and reliably feed and fire ammunition in small groups at point of aim. What else could you want?