JustinJ said:
.40 has a fair amount more stopping power than a 9mm.
No such thing as "stopping power" in a handgun. A 9mm +P runs just over 400 ft-lb of energy, most hot 40's are around 450 ft-lb. A 45 ACP +P will be just over 500 ft-lb.
A typical .223 will be just under 2000 ft-lb of energy and is considered to have very little "stopping power". There is a reason that they are illegal for hunting man-sized animals (deer) in most states. A 30-30 with just over 2000 ft-lb is considered minimum in most places.
In civilian use with good ammo there's not enough difference between any of the typical full size self-defense handgun calibers that anyone getting shot would be able to tell (military restricted to ball ammo is a different story).
Only things that will "stop" an attacker are loss of hydraulic pressure (bleedout), loss of electrical system (brain or spine hit), structural failure (broken bone), or psychological (mental decision on their part to stop what they're doing). Any of the handgun calibers being discussed are equally capable of causing any of the above results.
An 1/8 change in shot placement can easily cause the difference between a handgun round stopping someone or being totally ignored.
Here's a quick hint. Want to DOUBLE the stopping power of your handgun? SHOOT THEM AGAIN! Then keep shooting them till they're not a threat. That's the only kind of "stopping power" you're going to get from a handgun.
If you're using the Ultra-Zooper 99% one-stop nyclad-hollow-gold-talon ++++PPP and you really think a random hit with your fancy bullet is going to have a better chance of stopping someone than multiple center-of-mass hits with any full-size caliber, good luck to you.
Best thing the OP can do is get a handgun he likes in any of the major calibers and practice with it. Shot placement makes up about 99.99% of handgun "stopping power", the other .01% depends on the caliber .......