M&P 40c & G27 comparison:
Handled and fired examples of the M&P 9c and G26? The overall ergonomics of the M&P 9c makes it feel smaller and slimmer. Noticeably so with the smallest grip insert. (Too small for my hands.) The mag capacity of the M&P 9c is a slight advantage (unless you're restricted to 10-rd mags). The felt recoil of the M&P 9c makes for even better controllability than that of the G26 (which is a very good gun in that regard, in my opinion).
If I didn't already own a 3913, CS9, SW99 9c & G26 I'd probably buy a M&P 9c to go along with my M&P 40c. The 9c is a much better handling gun than the 40c from my experience with both.
Try examples of both, if possible, with some of whatever ammunition you commonly use in 9mm and see if one or the other stands out to you for your needs and personal preferences.