If I were still in a combat zone, or pushing a police cruiser, I would probably opt for higher capacity. The old adage, one can only have too much ammo when on fire or when swimming, sticks in my head.
But the truth is, most lethal engagements follow the rule of 3's:
3 seconds, 3 shots, and 3 yards. I like the .45 acp round for defense so I carry a single stack/SAO pistol that I am competent with and never have felt under prepared. Perhaps the fact that I carried a revolver for duty for so many years influences my thinking. Speed, power, and accuracy will carry the day, and maybe a little luck mixed in.
That said, my dream gun would be an MP5 in 45acp