Either the Glock or the M&P will serve you well. They will both take the higher capacity magazines that are used in the full size models, so don't worry too much about capacity. The thing to do is shoot both if possible. If not, at least get your hands on them and see which one feels best to you.
"They're on our right, they're on our left, they're in front of us, they're behind us; they can't get away from us this time."
Chesty Puller, USMC, Chosin Reservoir, Korean War
"You cannot exaggerate about the Marines. They are convinced to the point of arrogance, that they are the most ferocious fighters on earth - and the amusing thing about it is that they are."
- Father Kevin Keaney, 1st MarDiv Chaplain, Korean War