I was a Combat Arms Maintenance and Training (CATM) in the USAF. We instructed on and repaired all USAF small arms. For what it's worth and what I have seen I feel the M9 will break long before the M&P will. The part I saw break most often on the Beretta was the locking block. I have seen several dozen break.

I have a M&P and they are a much better firearm than the Beretta.