I bought a SIG P220 although the USP is nice, and I'm not completely biased as a result.
As far as magazine capacity goes, just what are you going to shoot that needs more than 7 rounds (and factory 8 round mags are readily available for the SIG) of .45, but not more than 10? .45 ammo is heavy.
Check out this 10,000 round test of the SIG P220:
http://www.galleryofguns.com/shootingtimes/Articles/DisplayArticles.asp?ID=1230
Here's what Gun Tests had to say in their 2001 review:
"Our SIG P220 (original review May 2001) came with SIGLite night sights, which helped it put 22 of 30 hits in the A zone in a Practical shooting test. We found the transition of the P220's trigger from DA to SA to be very predictable. Elapsed times were consistent, offering the control and perspective to complete the string of fire at the same speed every time. This exercise builds awareness and perception as well as precision, three key elements not only of sporting competition but also self-defense.
Another feature of the P220 that makes it an exceptional weapon is the layout of the controls. The Sig P220 prefers that availability of the slide release to the right thumb takes precedence, so that the weak hand can retrieve and load a spare magazine. Once inserted, releasing the slide need not wait until the left hand is in position.
The SIG's grip is plastic and wraps around the backstrap. The grip is at its narrowest along the line following the index finger connecting the trigger to the undercut of the backstrap. Although textured, the plastic grip is a little slippery, which would be fine if only the front strap were checkered. Grip tape on the front is an inexpensive alternative.
Slide and frame rails are full length, and you have to wonder what kind of alloy is used to make the frame assembly. At 14 ounces, it feels almost as light as a plastic frame. This adds to the SIG's appeal as an everyday gun, lending it to carry or driving options. "
Friends who have the S&W 4506 really like it. If you shoot the SIG, you'll buy it.