"So, which do you like better? The Glock or the M&P? Which one should I buy?"Okay, first, I don't know that I would say I really like either of them. The Glock is a lot easier to mess around in the guts of, if that's the sort of thing that appeals to you. As far as shooting goes, they're pretty much of a muchness. All these plastic cop guns are, really.
If one really sings to you, buy it, but you're kidding yourself if you think there are vast differences in performance waiting to be unlocked in one versus another.
The Sig P320? I think Bruce and his team have come up with a potential game winner in their X-Five, but that's not necessarily due to the mechanics of the gun as much as an otaku-like obsession with grip geometry, sight tracking, magwell size and shape, trigger shape and quality...
This is the sort of stuff that matters when you add up hundredths and tenths over the course of a ten stage match and probably doesn't matter dick across a convenience store counter or across fifteen feet of rainy midnight parking lot.
I think a plain duty P320 will work just fine in the latter situation, just like a Glock 17, or an M&P 9, or a VP9, or...well, really if you have a gun, any gun, that works, you're probably going to chalk one up in the "W" column.