I own both guns but in 9mm. My personal opinion about the Walther PPS is it is a great firearm that lives up to the Walther name but I have one major beef with the design. That is the safety mechanism built into the interchangeable back strap. If your unfamiliar with this feature, it deems the firearm inoperable by removing the back strap. As a machinist I feel the pistol has been over designed because of this feature with many complicated small parts. I have detail stripped my PPS but it is not for the faint of heart. Now as far as comparing the two; both my PPS and M&P have been 100% reliable from day one. The wider width of the M&P makes recoil significantly more pleasant. But the thinness of the PPS is more concealable. I prefer the M&P over the Walther and had I bought it first I would not own the PPS. I think the small size difference isn't enough to justify more money, less capacity (7+1 in PPS vs 12+1 M&P for similar length grips), and the stupid back strap safety that could fail down the road. Keep in mind this is coming from a lazy CCWer would usually just carries a .380 Auto in the pocket or IWB. But when I do carry a 9mm it is alway the M&P.
P.S. you may be one not bothered by such things but the PPS comes with a plastic guide rod where the M&P is steel. So that was an additional $80 I "had" to spend on the PPS.
Good luck on your decision!