I definitely agree with flop-shank in getting the holster for the full size version. The extra barrel length is a non-issue when carrying IWB. First of all, the full-size M&P is not that long anyway, second, the barrel is hidden inside the pants.
I have to caution you, though, that I find my full size M&P to be the hardest gun to conceal of all that I have. Harder than my N frame revolvers. Why? Because the full size M&P's grip is long and blocky. It prints like nobody's business if you wear it too far back or if you bend or twist the wrong way.
I can get away with it most of the time with a full size IWB, but concealing an M&P full size in an OWB holster is a challenge. At least for me,