I suspect that the $50.00 rebate and 2 free magazines S&W is offering didn't hurt either. In my experience the M&P45 is generally about the same price at the Glock 21 and slightly more expensive than the XD. When you factor in the $50.00 rebate, the S&W is cheaper than either, and with the free mags S&W is offering, you get four magazines for the pistol rather than two. S&W also offers a lifetime warranty (to be fair Springfield does too, although they don't actually manufacture the XD; Glock does not offer a lifetime warranty). S&W has an almost universal record for good service when it comes to warranty work, Springfield is hit or miss (I missed, I had an awful experience with Springfield warranty service).
I went with the M&P45 too. I've owned a Glock 21 in the past. It was a great shooting gun, but the grip was just too big. I always felt like I was holding a new bar of soap and the grip angle on Glocks has never worked for me. I've handled the Glock 21SF, and while it does feel slightly smaller, to me it feels like a bar of soap that has been used a couple of times, but one can still read the brand on the bar. I don't find the 21SF to be any kind of major improvement, although I will grant that it feels slightly smaller. It still has the God awful grip angle though.
I didn't even consider an XD45, I don't like the grip, the trigger, or the high bore axis on it. I'm also not interested in a grip safety.
On the other hand, the grip on the M&P fits me perfectly, and it offers interchangeable backstraps so it can be adjusted for different hand sizes. S&W also offers an optional manual safety, magazine safety, or both for those people who want them. Glock offers neither, and Springfield just introduced an XD45 with a manual safety at the Shot Show but does not offer a magazine safety. One can debate the merits of both all day long, but the simple fact is that S&W offers them because some people want them.
My IDPA shooting buddy and I both got M&P45s about the same time. Both have been totally reliable, very accurate, and a lot of fun to shoot. I couldn't be more pleased with mine.