I went with the M&P45 for several reasons:
1)The grip is much more comfortable than the Glock 21 (standard or SF) or the XD. The XD grip has never done anything for me at all in any of the models. The Glock 21SF feels like a bar of soap instead of a brick, but the grip angle and size just don't work for me (and I don't have small hands).
2)S&W has the best warranty, customer service, and is an American Company.
3)10+1 is 2-3 rounds more than the huge majority of .45ACP pistols hold, and 10+1 is both plenty of capacity for defense (IMO) and for any division in IDPA, where one is limited to 8+1 in CDP and 10+1 in SSP and ESP.
4)Price, my M&P with Novak Night Sights was cheaper than either the G21 or XD with night sights, plus I got two free magazines from S&W, giving me a total of 4 without spending an additional dime (OK, $0.41 for a stamp).
I couldn't be happier with my M&P45. I was first overall last weekend at an IDPA night match using my totally stock M&P45. My only complaint right now about the M&P45 is the lack of duty holsters available for it. I'd love to carry it on duty, though for a duty weapon I'd probably purchase a second one with the magazine disconnect.
It has never malfunctioned, fits me perfectly, is very accurate, and a lot of fun to shoot. I couldn't be happier.