There are a couple of things to keep in mind if you are selecting a handgun for defensive purposes.
First, handguns of any caliber from .22 short on up are capable of killing. But handguns of any caliber, loaded with any ammunition you can think of are poor fight stoppers. Why do we carry handguns? Because they are convenient. Because in our society it would be problematic at best to walk the streets and byways of America armed with an effective weapon. So we carry a handgun because it's better then no gun at all.
It's important that we understand this before we select what weapon to carry for defensive purposes. Once we accept the fact that no handgun is the equivalent of a Star Trek phaser, a plasma rifle in the 40 megawatt range or any other fictional death machine we can think of, we will be better prepared to make a choice of what handgun to carry.
It's generally accepted that with modern expanding ammunition, any reliable handgun in caliber .38 special or larger is adequate for defensive purposes. If you have unique concealment problems and still need to be armed, then you may want to opt for a smaller weapon, but you need to be aware that you are arming yourself with a weapon that is on the low end of the scale for it's ability to stop a fight and that nothing at the other end of that scale is very effective either.
Pick the handgun that fits you the best. It doesn't matter if it's a revolver or auto, 1911 or Glock, .38 spc, 9mm, .357, .41, .44 .45. All that matters is that it is as close to 100% reliable as possible and that you, not me, not an IPSC champ, not your next door neighbor, YOU can shoot it well and that it fits your lifestyle. The $3500 custom 1911 man killer with the brass knuckle trigger guard and the bayonet lug with the optional 16" sawtooth bayonet isn't going to serve you well sitting in your safe or on your night table because it's too bulky and heavy to lug around. You must be aware that you may have to modify how you dress to accommodate carrying your defensive weapon.
Once you have chosen the handgun that fits you and your normal manner of dress, invest in good professional training and a lot of ball ammo to practice with. It's software, not hardware that wins fights.
When you are familiar enough with your chosen weapon that everyone think you carried it from the womb, then you are ready to worry about picking defensive ammunition.
What it all boils down to, is that you will win or lose the fight, the tools you use are largely irrelevant.
Jeff