put two experienced shooters 10' apart. In a 4 second span, there is no retreat, there is no one armed defense. If your first shot does not remove the threat, there is a 40% chance that you wil receive the next hit.
If you are not an experienced, under stress, shooter, the probability swings to the intruder.
Great, you have a mantra for one of the 5000 different variables of deadly threats out there. Most SD scenarios won't go down OK corral style. You will likely be ambushed, your attacker will be just as scared as you are as soon as the first shot is fired, even experienced shooters will likely not achieve a 50% hit rate @ 10' in a SD scenario, trigger pull will occur until someone drops, your attacker may not be armed with a firearm but a knife, you pinned an in a shootout @ 50' with the opportunity to utilize cover, etc. etc.
Not to mention the fact that I'm not going to just sit still, draw, and fire. You may have to, you may get in a SD situation in a phone booth, but movement is your friend. You may get tired of carrying that 4 lb 6" barrel six shooter and decide not to carry it. You may not have the money to afford ammo for it.
I agree with one thing. If you're going to carry that .357 or .44 every single time, even though you're tired and you don't feel like strapping it on, then that is the better choice. Well, assuming that you've practiced the manual of arms enough to be proficient. No arguments from me. .22 in the hand is worth better than a howitzer in the safe. If you're gonna carry that howitzer and be proficient at it... then go for it.
Now, am I trying to argue against carrying a hog leg and against big calibers? No, not at all. Note I said that "I think the best caliber is the most powerful round that is readily available, can be controlled by the operator proficiently, and is offered in a reasonable firearm that combines size/reliability/weight/recoil charecteristics within the shooters desired range.". What I'm trying to say, however, is that you're much better off to be proficient with many rounds of .32 or 9mm than you would be to practice once a year with .44 magnum and be limited to 6 rounds. Every man has a reason for carrying what they carry.