I couldn't decide so I picked both up - Glock 22 and SAM 1911 commander in .45 ACP. The .40 S&W caliber is snappier but it is fun to shoot. Shooting the .45 ACP just awesome just because it can spit out that big chunk of lead 
If it was good enough for John Moses Browning its good enough for me
actually, 38 acp was good enough for him...he only went to 45 to sell it to the army. And they only wanted it to shoot horses with.
To me if it is M&P must be 40SW & 45ACP must be 1911.
.45 acp brass is cheaper and easier to find! As a reloader, that's important
Not big Hands I liked my colt with the single stacked mags - fit my hand well - the S&W MP 22 fits me fine so how much fatter is the 40 or 45 over the MP 22 ?s RJ
So what would be your choice in a S&W MP pistol - the 40 S&W cartridge or the good old 45 ACP cartridge - Pros - Cons ?s
RJ
Very little difference between the M&P22 and the 40, and you can fine tunes the grip size with the backstraps. The 40 with the medium backstrap is almost identical in size to the 45 with the small backstrap...so the whole question is basically a non-issue.
I like both the 40 and 45 about equally (and have both). If you can only have one or the other, base your decision on the cost and availability of ammo, magazines and other accessories...