I guess if I had to sum up my argument, it would be this: you can debate power level and "knock-down" all you want, but at the end of the day, once you have "enough" caliber to do the job, more isn't necessarily helpful, and can be harmful. It's generally accepted that 9mm and above are "enough" caliber for defense, as long as you're shooting +P and hollowpoints, is my understanding. I have also heard some folks say that the newer .380 loads are to the point where they'll do the job as well, but anything less than that, and you're making serious compromises. Are there cases where the extra expansion of the .45 would have nicked an artery or the spine, and a 9mm would not have? Sure. But those are edge-cases. In the majority of shootings, there are a LOT of other factors at play, other than caliber, that determine whether the defender successfully stops the attacker. If I offered to shoot you in the face with a 9mm or a .45, which would you take? Uhh... neither, thanks! So the focus on caliber, although fun to have over a beer or whatever, is kind of wankish.
What are those other factors? The single biggest one, hands down, is training. As was said earlier, if you miss with a .45 or a .22, it's still a miss. Give me the choice between shooting 5000 rounds a year with .45 or 10000 rounds a year with 9mm for the same price, and I'll take the latter. Ability to control the gun is another. Lots of people are going to put more rounds on target faster with a 9mm than a .45, because of the reduced recoil. And let's not forget situational awareness. How many fights are over before they start, because the good guy's gun never even left its holster, because he didn't see the threat soon enough? Okay.
Now, I've gone off on a bit of a tangent, because the OP asked specifically about caliber, but since the conversation has strayed onto other, related topics, that's my two cents.
