My take
I am interested in how they compare in the areas of trigger pull, accuracy, duriability-reliability and general shootability.
I have the HK USP 45F and it has been flawless in 4000 rounds. I had a 226 in 9mm and it to was flawless. I choose the HK because I wanted a 45 with more than 7-8 rounds. plus I heard about HK's reliability. After renting one at the range, I was sold. I bought one the next day and have never regretted it. But, in .40, I don't know.
the way that I see it (keeping the above in mind):
Trigger- the Sig is the better pull, hands down.
Accuracy- they are most likely tied. My USP and 226 both grouped less than 1.25" at 25 yds from a rest.
Durability/ reliability- Again, tied-I don't think that you could wrong either way.
Shoot ability- Honestly, I have to give the edge to the USP here- The 226 always felt better in the hand, but the USP always felt better shooting. It just seemed to be easier to shoot quickly and accurately. But again, that is my experience (2 different calibers, though) your mileage may vary.
Other things to consider- The 226 is da/sa/ decocker. The USP is da/sa/decocker and has a manual safety. I like manual safeties and the ability to carry cocked and locked.
Both are great designs and I don't thing that you could go wrong either way. Try them both and then decide.