advantages to the USP are:
integrated rail mount
polygonal rifling
can be carried cocked and locked
can be changed to DAO, with decocker, without deckocker, C&L only, LEM, spurred hammer, bobbed hammer, etc.
abidextrous mag release
can have ambi safety/decocker
captive recoil spring makes assembly easier
when i was buying an SD gun, it came down to the same two guns. 229 or USP compact. (both .40) i got the USP compact and have no regrets at all. i liked the Sig, and still rent it on occasion to see if i'm still happy with my decision and i haven't waivered a bit. just like the fact that the USP can be changed to suit ones taste and is more ambi friendly. you can't go wrong with either but i'll cast my vote for the USP.
Bobby