Well, I have a USP .45 Tac, and my 2 good shooting buddies/neigbors hae a USP .45F and a Sig P220ST. We bought them all at the same time:-).
Anyway, both are AWESOME guns. I'd like to get a P220ST some day as well. The USP DOES have a few advantages though. . .
For one, the extra 2-4 rounds capacity. Another, the ability to carry cocked + locked, etc. -I can't stand not having a manual safty (on p220). (mainly for caring cocked).
Both guns shoot awesome, though the USP's are generally a tad more accurate. The biggest thing is this: With the sig P220st, after firing around 250 rnds, the side doesn't quit go forward when chambering a new round. It get's to gunked up, and you have to clean and lube the feed ramp, chamber, and slide rails. Then it's fine again. Now, on my Tac, I've fired over 1000 rnds w/out cleaning it (multiple times), then decided to clean it simply on account of the fact that I hate to neglect a $1k gun. (or any for that matter). It was still working as well as the 1st round after cleaning. I've also fired all 3 in the snow, rain, etc. . never encountered any problems in adverse weather. . .
Last thing, try them on for size. . my friend with the p220ST bought it becaus the USP felt uncomfortable in his baby (err, I mean small) hands. . . I'f you can't shoot it comfortably, don't get it!
Later,
Justin