TONS of factory and aftermarket replacement parts for the Glock (you can literally build a complete pistol with the amount of aftermarket parts out there), and they are cheap, easy to find, and even easier to install. Can you even buy factory parts for the M&P yet?
You can customize the Glocks trigger pull with either an 8lb, 5lb, or 3.5lb connector as well as adding the NY1 or NY2 trigger springs for a different feel. With the M&P you are stuck with the factory trigger.
Warranty support for Glock is top notch. Warranty support for M&P? Ask GreenTips about that, his story isn't the only S&W warranty nightmare I've read. There are a number of complaints re: very poor warranty support here in Canada (go straight to the dealer you bought it from, don't even bother with S&W), people sending a gun back 2 or even 3 times and it's not fixed, etc, etc....
You can field strip a Glock in about 5 seconds and with NO tools. The M&P takes longer due to the fact that you need a tool to disengage a small part inside the slide, which, I'm sorry, is extremely LAME. That tool is stored in the rear grip of the gun (it twists in). Will the slot that holds the tool in (you push it in and twist it) wear out over time? Not sure, I guess we will find out. Just more moving parts to fail IMO.
Glocks have a stellar track record for taking abuse, and lasting forever. M&Ps are still new.
You can get cool 10/17 pinned magazine for Glocks for $40 here in Canada (there are even aftermarket magazines now for a great price from Dynamite Monkey which come with a free double mag pouch if you buy 2 mags). I don't know of anyone who sells pinned M&P mags if they are even available up here, and the normal ones are usually about $50 each.
Some places sell the M&P with those abominations some like to call safety devices (ie. internal lock, and magazine disconnect), so if you get it from WSS you'll have to ask if your gun will have them, and they won't have the slightest clue what you are talking about. Glock, no such blatant stupidity.
Glock Tenifer finish has, again, a stellar rep for not rusting. It's fairly common to read about people having surface rust problems with their M&P's. Might have been resolved, who knows.
Etc, etc, etc, blah, blah, blah..... LOL
Just my 2 cents. All that said, the Glock may not fit you. If it doesn't... get a 1911
My favourite is the G17, and I mean really..... what bad things can you say about the G17? It's probably the handgun with the best durability/longevity record that has ever been made.