Use enough gun. Your carry or defense weapon should be the largest caliber in the smallest size that you can shoot/use properly.
I've read a gun magazine article where the writer said to carry the largest gun you can to "scare off" or intimidate a possible attacker.
This is BS. A real gun fight follows what I call the 3x3x3 Rule; 3 rounds in 3 sec at approx 3ft. You should plan to draw & fire in a lethal force incident.