Fine guns both of them and you really can't go wrong. As to specific features - the P226 is heavier, but all metal; the USP is lighter and has a polymer frame. In my opinion, the SIG trigger is better; I find the H&K DA trigger to be particularly bad. When it comes to carry, the most commonly found version of the USP can be carried in Condition One (cocked & locked) or decocked, as you prefer. There are several other versions as well. The P226 (outside of a new SAO version) has only a decocker, no safety.
I'm happy to have both a SIG (P6) and a USP (.45) in my collection.