XD vs M&P (or Glock, FNP, etc):
Almost all $300+ polymer pistols are in fact really functional, reliable, and low maintenance firearms. It should be noted that there are a handful with not so great triggers and really just a very select few with reliability issues (namely Taurus)...most are excellent. Even most Taurii are pretty solid. I like the XD/XDm trigger the best, and the S&W might have the best all-around ergos (for me, that is). I value the minimal weight and low bore axis of Glock, but the rest of its design leaves me a bit frustrated. Those who opt for FN polymers often prefer a hammer-fired weapon and/or ambi controls.
Buy the one with the fit, features, and finish you prefer. PERIOD. The rest is nonsense.