I agree with the posts pointing out the importance of shot placement. The one shot stopping power thing is mostly for manufacturers to sell ammo and shooters to talk about IMO. I think much of the under powered knock on 9mm is based on older FMJ ball ammo and in that case it probably is true, but the modern powder, firearms, and especially bullet technology make that comparison apples and oranges. You can find stories of cops shooting a drugged up perp multiple times with 9mm and he kept coming. But I think it is a mistake to automatically assume that if they had been shooting .40 or .45 or 10mm he would have gone down like a sack of potatoes. Maybe, but probably not.
I carry 9mm because I have confidence that premium self-defense ammo will get the job done, because of age and arthritis I like the lighter recoil, and I wanted a gun my wife could handle, also mainly the recoil issue. I carried a 1911 in combat and loved it. But I am pretty sure that any situation I cannot handle today with 9mm would probably be a problem with any caliber.
To date my concealed handgun has stopped 1 armed robbery attempt. The instant he realized I was armed he dropped the weapon back in his pocket, showed me his palms and hauled butt. Not sure what he had, he kept it in his hand in his pocket until he decided he was not a gun fighter. I am so glad he made that decision for both our sake. But obviously the most decisive factor was not caliber, but the fact that I brought a gun to the gunfight. That is rule 1.