IMHO, it boils down to training. even if you carry a .22 if you are well trained to shoot it can do the job.
Let's just say ok you decide to get the .45 but if you don't train and get yourself and familiarize with the pistol, ( how to shoot, recoil, dry fire... not just pulling the trigger but holstering and unholstering, etc ) you carry, then my friend that firearm is just an expensive paper weight.