If you're set on maximum concealment, just keep the carry mag's standard flat baseplate on it. That little pinky tab really does make a difference in the overall height of the M&P9c vs. the G26.
On your belt, you'll probably not notice the extra weight vs. that of the G26 unless you're on a day-long hike.
-J-
Ive got an M&P 9c, and I know its bigger than a Glock 26. I carry it in an MTAC with a Bullhide belt, and dont have a problem with that. What Im wondering is, assuming the same holster and belt, is the 26 smaller enough to make a difference in comfort and concealability?