The best answer is - The gun you shoot the best with.
The caliber matters not if it is one of the known "man stoppers." 9mm, .40 S&W, 45 ACP, .357 sig, .357 magnum... there are more.
Different guns have different amounts of kick. A .22 doesn't have much, a .44 magnum has quite a bit. Then there are a ton in the middle.
Silkiechicken has a 9mm Springfield XD subcompact that is her primary gun (though it is my gun).
I shoot a Springfield XD 45 compact, which is my primary gun.
If this is for an inside the house gun, I'd say get as much gun as you can. There is no issue of concealment, so a full size makes sense. Then pick a good caliber and a model that feels good to you (grip, grip angle, safety, sights, looks, etc) and you'll be good to go. Talk to friends to see if you can shoot their guns, go to a range that rents them... Try before you buy!
edit: Barrel length matters somewhat. The advantage of a shorter barrel is that it is easier to conceal. That's about it. A longer barrel is more accurate due to the longer sight radius (the distance between the front and rear sight), and the longer barrel allows more powder to burn, which leads to faster muzzle velocity. The speed of a .45 out of a 3" barrel is slower then out of a 5" barrel.