Pardon the interruption but are any among you so terribly challenged as to suggest the M&P is at all difficult to field strip? Remove magazine, verify empty chamber, engage slide lock, rotate takedown lever 90 degrees, disengage slide lock and pull trigger. Both models are simple and hardly a point of contention. I suppose if it were, for a full strip you'd need that rusty nail for a Glock while the Smith uses a long steel pin to capture the grip panel that could be pressed into service as a roll pin punch, with a built in handle I might add. Not a likely scenario but neither is finding a rusty nail in the desert.
Secondly, if the OP wishes to: add a match/ported/threaded/conversion barrel, compensator, sights, textured grip panels, fire control parts, extended magazines, variable spring weight uncaptured guide rods, holster of any style imaginable, they are available for the M&P. If his requirements extend beyond those items then apparently Glock aftermarket is the way to go. I cannot at the moment think of something not on that list that is unique to Glock save perhaps a new frame I recall being offered in alloy or steel.
They both work very well yet we rehash this same debate any number of times every month with the same arguments put forth. Find what you prefer and learn it are the basics. What is preferred may be OEM sights vs. additional cost of aftermarket, grip angle, country of origin (the most limiting to someone who appreciates firearms), availability (for seeing, touching, trying firsthand) and extractors which are more serviceable on Glocks but have not to my knowledge been problematic on M&Ps. The rest seems little more than fanboy fodder that seeks approval from others for one's personal choice.
My vote would be a Gen. 2 or Gen. 3 Glock or any current production M&P with a preference toward the Pro Series, barring any other caliber or manufacturer suggestions.