As noted, the M&P was originally designed as a .40. The slides have differences besides breech face. The 9 mm has a long central rail under the rear of the slide ala 1911 and lightening cuts forward of the chamber area of the barrel. The .40 has no lightening cuts and the central rail is modified to allow 15 round capacity. There's just a stub rail to pick up the rounds from the magazine.

Pretty much everything else is the same except for roll marking on the slide and magazines.