Excellent points from the Cap'n.
The most important characteristic of a defensive firearm is reliability. In my opinion the next in importance is selecting a gun that you will actually carry. That varies with the individual based on their build, life style, manner of dress, tolerance for pain, mode of carry, need for concealment, etc. Finally the other factors go into the choice: cost, power, accuracy, fit, and so on. But first and foremost you want a gun that will function properly and secondly one that you'll have on you and not sitting in the safe at home after you learn how inconvenient it is lug around every day and everywhere you go.