A large part of my decision was customer support, and availability of aftermarket parts.
In my experience Glock has been awesome. Research the support and QC of the M&P (check out the mp-pistol forum on the net and on CGN, appears to still be going through some growing pains) and the XD (do they offer any parts yet or do you still have to send the entire gun back to the factory?).
Glock parts you can get dirt cheap, and you can take detail strip the gun down to it's smallest parts in about a minute with nothing more than the inexpensive Glock tool (which is really just a 3/32" punch). Glockparts.com among other parts suppliers will ship most parts to Canada for about $3 and they are dirt cheap to begin with. The fact that you can get 17 round mags pinned to 10 for the Glock is also super cool.
My dad didn't like the grip angle on my Glock when he first shot it. He kept saying "ahhh... I can't get used to this, it doesn't feel natural, I'm used to my 4006". Then 2 magazines later he was shooting groups with my Glock that made my groups look like I was shooting blindfolded. He then gave me the "I guess it's not so bad after all, just take some getting used to" with a bit of a smirk.
Glocks have also been out a long time and have worked on getting rid of all the problems they experienced. I wouldn't mind getting an M&P (to play second fiddle) after they have been out for a few years and worked out all the kinks.
Glock for me is number one and an easy choice!
Get one from Mark at CanadaAmmo! Cheapest price, and free shipping!!! Great guy to deal with! Put up with my million questions about both the Glock and M&P!