With today's ammo, caliber has lessened in importance. If ballistic gelatin tests are any indication of stopping power, ammo choice is critical. Remington Golden Saber, Speer Gold Dots, and Winchester Ranger ammo will do a good job of protecting you. Choose caliber according to ability to control it. Most state police agencies use .40 caliber. Second in use is 9mm. Only 4 states use .357 Sig. 30,000 plus New York City officers are issued 9mm.
I used to be a .45acp man. Before that I was a .357 Magnum man. Now, after generations of shooting, my two favorites for confidence in carrying are 200 grain .44 Special Speer Gold Dots, and 127 grain Winchester Ranger 9mm +P+. My reasoning is that the .44 can't be improved. It surpasses even the .44 Magnum in stopping human beings. I like the 9mm because of the bullet design and the fact I will not run out of ammo.