I was taught that a 9mm is a better defensive firearm because the recoil management is easier, the ammo is less expensive (meaning you are more likely to practice) and the differences in wounding capacity were minimal.
I was at an outdoor range recently, having just finished a class, several of us were “playing”. Side by side, me with a 9mm Glock 19, my friend with a full size .45, 10 rounds, from high compressed ready, on an “UP” command, who would go to slide lock first? Ok, I do practice speed shooting and have very little muzzle flip with my 9mm. However, my friend has upper arms like small trees, and has relatively little muzzle flip. What do you think happened?
I won by about 2 shots. I was back in HCR before he went to slide lock. Even looking at accuracy, we were about even there. So, reload 10, and switch guns. I’d never shot his gun before. Looking at the video (I see You Tube in the Future for this) I missed the “UP” by at least one shot, didn’t hear it. However, I went to slide lock just as he was coming back into HCR.
So, what did I learn from that experiment? Ok, maybe not scientifically valid, but FUN! Practice seems to have made the biggest difference in the outcome. I’m not close to being as strong, but have more time speed shooting into a high center chest area on a target. So, my practice beat his superior strength. He was really fast, as well, just not quite as fast. He also didn’t notice a big difference between the 45 and the 9mm. Remember the tree trunks? This is a man who doesn’t see much muzzle flip until the Desert Eagle comes out, and no, I respectfully declined that chance to shoot that. My hand was already bruised from the buckshot in the 12 gauge (told you we had fun).
Bottom line, IMHO, your best defense is the firearm you are willing to practice with, to get to know and are comfortable with. With a caveat, I wouldn’t suggest anything smaller than a 9mm.