The bigger calibers will give you more knock down power but dead is dead whether it's a .22, .45ACP or 10 Gauge.Killing is not the issue.
Stopping the attacker is.
A small caliber well-placed may kill an assailant, but it may not stop him from killing you first.
His dying on the lawn six hours later, or the next day in a hospital does you little good.
One shot in the head will stop him his tracks, not on the lawn for 6 hours but dead on the floor. Personally I don't aim the pistol unless I plan on killing what is front of it.