This debate is endless I think..lol...its been going on as long as I can remember. Having changed opinion many times I finally came to the conclusion that the difference between caliber's is less important than other factors. For defensive style shooting the factors that I consider are:
concealability
function--including reliability, unusual movements required to get into fire mode.
accuracy combined with speed of use.
speed of followup shots.
ammo capacity
speed of reloads
----with the metallurgy and bullet design provided to the average consumer today any caliber from 9mm on up can be made to perform to your desired critieria, (deep penetration or rapid expansion), and for me when you consider those two options each caliber handles the recoil pulse slightly different at different power levels and that is an important factor in my ability to have the firearm/ammo combination fit my needs.
----bottom line is --it just dosent matter, every combination has pluses and mineses for each individual. I would tend to believe that most shooters would be adequately armed with any pistol in a 9mm or above caliber and commercial ammo--and then training comes in to get the most from the combination that you are using........fubsy...