I try not to advocate one pistol over another here (at least these days). I find pistols are very subjective when it comes to shooting and most are roughly equally reliable when talking about the ~$500 striker fired pistol.
Since you asked, my personal reasons stem from a few factors. I like the fact that I can look at a Glock and tell that the striker is cocked. If the trigger is forward then the striker is cocked (it will stay to the rear when the striker is released). On an M&P you have no such visual option. Coming from hammer fired pistols it's a quirk of mine that I like that visual cue.
I find I shoot a Glock with a stock trigger essentially just as well as an M&P with an APEX trigger slow fire, and actually better when I pick up the pace. All else being equal, I'd prefer to use a pistol in the stock configuration when it comes to self defense. On Glocks I add Talon Grips, replace the magazine release, sights, and I polish the trigger bar, but that's it. I don't mess with the firing mechanism or trigger pull weight. On an M&P I always put in an APEX kit (though the newer M&Ps really don't seem to need it). Massad Ayoob has a number of articles about modifying firearms and the legal implications. The general result is that as long as the modifications don't lead to you claiming you "accidentally" pressed the trigger when you didn't mean to, you'll be fine. I also live in a very pro self-defense state. However, I still prefer not to go down that road if I can and in this case all else was actually equal (it usually isn't).
Though these days I carry a Glock 26 with the factory +2 baseplates, which is very similar to the M&P 9c in terms of size, I do at times carry a Glock 19. I find the slightly smaller size makes a difference to me when it comes to concealment compared to the fullsize M&P.
The last factor was I personally have had QC issues with a number of M&Ps. I had an M&P 45c that would fail to feed when you fired more than 50 or so rounds. I had an M&P 9 fullsize where the trigger was malformed and rubbed on the frame during the press creating a heavier than normal trigger pull. I had an M&P 9c that ejected straight to the 6 o'clock position and right into my forehead. I had an M&P Shield where the trigger safety would not engage between shots (it fell under the recall that happened). Lastly I had an M&P 9c where at around 800 rds the slide and barrel started peening and the sear housing block wouldn't sit tightly in the frame causing it to tilt (I had to expand the trigger bar loop to account for the off chance where the trigger bar was pushing the housing up at the same time as actuating the sear otherwise the striker wouldn't release).
Now, all these issues were fixed by S&W. But it's been my experience that they use their lifetime warranty as a form of QC rather than being more diligent before the pistols leave the factory. There will be many folks here that have had no such issues with M&Ps. I'm a statistician by trade and one man with a few samples isn't enough to judge a whole model line and that's why I gave S&W multiple tries. But what made me stop in the end was the variety of issues I kept finding. I have had issues with Glocks in the past with ejected cases bouncing off my head (none so bad as that M&P 9c) and generally less than ideal ejection. But the firearm still functioned, using "hotter" ammo (such as self defense loads) generally made those problems go away, and that was typically the only issue so I knew to look for it (I've owned easily half a dozen Glocks as well) and I have a few tricks to try to remedy that problem if it arises.
Given the issues I had and the fact that I shot Glocks as well or better made me switch from M&Ps to Glocks. However, this is just my personal experience. I'm of the opinion that each shooter has to find what works best for him/her.
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