I was going to have a long, drawn out post on stopping power, but this summarizes my point in its entirety:
Get a gun/cartridge combination that goes bang every time you pull the trigger and which you can shoot quickly and accurately. No common defensive handgun cartridge will quickly and reliably stop a human being who is committed to causing serious bodily harm, unless the bullet from that cartridge is applied to the correct anatomy. When the bullet is applied to the correct anatomy ANY of the 3 will serve equally well (9mm, .40, .45).
Get gun in a caliber of 9mm/.38 or larger. Shoot gun lots and lots. Problem solved.