This is an age-old issue, these days usually described as the "fast and small" versus "slow and big" positions, So, a few thoughts from an ex-15 year cop and forensic expert:
1. No handgun bullet can ever be counted on to absolutely stop an attacker, especially with "one shot," as a few current, opportunistic gun writers like to expound upon. This is a matter of basic physics - for every action, there is an opposite and equal reaction - and if a handgun were capable of literally knocking down the attacker, the shooter would be on their butt also.
2. That said, since incapacitating an attacker is partly due to the amount of damage done to vital tissue/organs/arteries, etc., a bigger bullet:
a. will have a better chance of hitting something more vital than a smaller one, especially if it's a hollow point that penetrates deeply and expands reliably at a wide range of velocities, which today's bullets by major manufacturers are designed to do;
b. will be heavier, and the extra weight (i.e. a 230 grain bullet in a .45 versus a 124 grain bullet in a 9mm) will have a greater ...er..."impact" on the attacker than the smaller one.
3. The .45 has proven over about 100 years that it offers the 'best chance' of stopping an attacker as quickly as possible compared to other rounds; even .45 ball ammo (non-hollow-point) has proven effective in combat, if for no other reason, because of its size.
4. As is correctly asserted by knowledgeable authors and gun pundits, accurate shooting, via PRACTICE, so one can place shots where they will most quickly incapacitate the attacker, is the most important facet of all this: after all, even a .45 through soft tissue that does no real damage is not as useful as a 9mm through the heart.
5. Nonetheless, as a former police officer, and current CHL holder myself, I carry either a .40 caliber Glock (23) or a .45 (Ruger P-345D), because, all things being equal and assuming a reasonably accurate shot with any bullet used, those cailbers give, in my opinion, a much better chance of quickly stopping the attacker.
6. And remember...a determined, or drug influenced, attacker may certainly NOT be stopped by one shot of any caliber - even a shot through the heart, which can still allow them nearly a minute to function before collapse. Therefore, if your life is truly threatened and you have to use deadly force, keep shooting until they stop.
- knightshadow.
I'm saving your entire post, and am going to forward it to my dad as I think he'd love to read it as well. Thank you for putting it into terms I can understand. Sometimes I try to follow these threads and I get delirious. :D