Zentao -
I have a 228 and a USP45C (I'm in the market for a USP9C), so I can't give you a direct comparison. I like both guns, neither is a bad choice, both are approximately the same size, fit my hand well, shoot nicely, accurate & reliable, but I will say that for me, the balance of the SIG is better (polymer frame guns tend to be top heavy) and it's a little easier to carry in that it is more rounded and a bit flatter/thinner. OTOH, the USP is +P rated, and IMHO, probably the more rugged gun of the two (rugged being a relative terms, SIGs are plenty tough).
RE: 226 mags, yes, they fit & work just fine, though they stick out a bit. If the availability of hi-caps is an issue, the USP is a 1993 design, so finding legal hi-caps will be a problem (and $$$$ if you can even find them). SIG factory hi-caps are available, though pricey. I've had good luck with both 228 and 226 hi-caps from ProMag ($14.95 @ CTD), though I would suggest replacing the OEM springs with Wolffe's +5 or +10%ers. They've worked fine for me at the range (I use factory mags for carry). HTH, good luck, M2
[This message has been edited by Mike in VA (edited January 15, 2000).]