Concentrate on the caliber you can use the best and be more accurate with. Thats the size that will protect you. I don't play the "my bullet is bigger than yours" game. If you have a .45 and can't control it, what good is it? The best SD caliber for any individual is the one they can shoot the best and be the most accurate with. Some people can be accurate with many different calibers some maybe only one. But what ever you do choose, practice with it often, it's not like riding a bicycle, with guns you always need to practice not only shooting them but keeping them clean and in good mechanical shape. Just my 2 cents.