If you are going to buy a 9 mm and then go looking for +p++ ammo, get the .40 instead.
Tom:
It might also make sense to buy several more of those 9mm's in this case. The M&P guns can take only so much abuse

....
There probably is a little to be said for +P loads, but at some point it becomes bragging rights. A good SD load in anything nastier than a .380, presuming a solid hit, and no winter clothing issues, should be more than enough to ruin the BG's day. "Plastic tip" SD loads should solve the clothing problem, leaving only the issue of a follow-up shot, and that's got too many variables to really predict.
(That said, go with a caliber/gun mix you can handle well enough to get in that follow-up, should it be needed. IMHO, those ultra-light .357's, for example, are a "where the hell did my hand go?" situation for most shooters after the first round, even if you can stand the pain....)
Lest I get nasty flames, the newer SD round for the .380 aren't bad, either, but the guns are generally not fun to shoot, and that's part of the problem: If you can't comfortably shoot the thing, you won't practice with it, and the best you can expect, besides luck, is a nasty flinch. The 40FS and 40C are a little snappy, but comfortable for me. So's the 9mm, but given the mix, the 40's a better choice
for me. "Outside", it's an Officer's-sized 1911. Actually slightly better for me, but the M&P is a better carry in grubbies.
IMHO, the .40 Shield would be about the same as the 40C - maybe a little less comfortable, but I've yet to try it.
Buy several of each....
Regards,