See thats the $64 K question!
I concluded the actual type or caliber of the firearm comes second, compared to shot placement, how many and how fast. This combined with practice and the correct mindset to survive no matter what happens.
Saying that, I still use "effective" calibers in my carry guns. I think a lot of folks out there, buying big bore handguns and rarely practice with them, actually have gotten them into a fake sense of security.
Also, a lot of new CCW/gunowners, often times "outgun" themselfes. Imagine someone with little or no training at all, shooting a S&W mod.19 with 4"barrel loaded with 110 gr. JHP 357 mag. in a hallway in the dark. If they don't hit with the first shot, they can't hear or see anything for 30 sec. or more.
My primary carry gun is a GI Mod.1 in 50 GI caliber loaded with 230 gr. CHP at 1000 f/s. It rides in a Safariland paddle holster mod.568. Secondary carry guns. Glock 19, Kel-tec. P3AT, S&W M0d.642, GI 1911 NN and Ruger Vaquero for the trails here in AR.
The buttom line, is that I believe in practice and being prepared.