I'll play devils advocate for a minute. The M&P metal frames appear cast, and as was already pointed out - aluminum. That isn't necessarily a bad thing, so long as the casting process and metal alloy has been perfected = not porous/susceptible to cracking.
I started looking at the metal M&P's. That stuck out to me as my philosophy with frames is "metal" absorbs recoil better than the composites.
That said, I consider "metal" to be stainless steel. When you dig in to the weight of the metal vs composite M&P's there is not much difference in weight because of the aluminum.
In practice, which shoots smoother with less recoil? Probably the metal, but by how much? To me, on the outside looking at the numbers I can't justify it.
For the added price - I can get more bang for the buck with the classic composites.
For what it is worth, in 9mm my favorite pistol is a Kahr MK9. It is the best shooting small pistol I have - which is why I bought it back in the day. It is heavy stainless steel and a pretty small pistol. It is more concealable than the majority of my pistols (aside from the Bodyguard). What it doesn't have is round capacity - 6 rounds flush mag, 7 and 8 with the extended mags. I switched to the Equalizer late last year as my primary as its a double stack and I really like the form factor. That doesn't discount the MK9, it still has its place.
As to the metal M&P's - go stainless steel and add 10oz or more. Then I might look at them again. Though, I am sure the casting process would be vastly different with that metal and trying to machine it may not be possible.
