Preparedness, mindset, the ability to place a shot correctly, under stress, in time.
Everything else is meaningless if you don't do those things. I never lose a minute of sleep or worry about whether the bullet is going to expand for sure. Some Pros only carry ball, because they value function above all else. I have no argument with them.
Use any proven defense round, or plain old ball ammo, as long as it feeds 100% in your gun, and worry about the other things.