weum817, we T&E'd the M&P40 against the 4th gen G22 to replace our 3rd gen G22/23/27s. The M&P broke down during the first mag and once thereafter. The G22 went 8000 rounds without cleaning and had no issues. We're staying with Glock.
I've had 1 G19 that had rust pits all over the slide-don't know what caused it, but it was horrible. I also ran into a M&P40c that rusted all over the slide in a dealer's case. Overnight. Sometimes metals just rust, something gets on them and causes a reaction.