I had both, and sold off the Glock. My M&P 9 Pro has been nothing short of stellar. Not a single FTF, FTE, or any other kind of failure whatsoever. Spookly accurate. Feels great in the hand and points very naturally. The trigger in the 9 Pro is a night and day difference from the Glock. Light, clean reset, and breaks beautifully. I really didn't like the Glock trigger, how it pointed for me, or how boxy it felt in my hand. But that's just me.
Sure, Glock was the original, but look at how they've changed after the M&P's came to the market. Both are dead nuts reliable, and have massive fan bases. Personally I prefer the M&P, and can't see myself purchasing another Glock, but really, you can't go wrong with either.
Cal.