A pretty good and informative study of handgun caliber effectiveness. Here is the conclusion at the end if you don't want to read the whole thing:
Conclusion
This study took me a long time and a lot of effort to complete. Despite the work it took, I'm glad I did it. The results I got from the study lead me to believe that there really isn't that much difference between most defensive handgun rounds and calibers. None is a death ray, but most work adequately...even the lowly .22s. I've stopped worrying about trying to find the "ultimate" bullet. There isn't one. And I've stopped feeling the need to strap on my .45 every time I leave the house out of fear that my 9mm doesn't have enough "stopping power." Folks, carry what you want. Caliber really isn't all that important.
Take a look at the data. I hope it helps you decide what weapon to carry. No matter which gun you choose, pick one that is reliable and train with it until you can get fast accurate hits. Nothing beyond that really matters!