I read and talk to people a lot, see a lot of people shoot, but have never had bullets coming at me (on purpose) so, FWIW:
For around 20 years I have cast a suspicious eye on the many pieces of Total Proof that bigger handgun cartridges are notably better. Even though no one (much) quotes e.g. Marshall and Sanow anymore, their overall gist and their anecdotal methodology (bullshit, lies) infuse many of the conversations. No one much says stopping power or one shot stop, but they still mean it when you get into it.
We even had a couple wars in the last 17 years, and have gotten stories (here even) of those who had to use ball in their 9mms and made it out to tell the tale, despite the assurance from the all-calibers-that-start-with-a-four crowd it's a peashooter.
Handguns are mediocre at their jobs. All of them. Changes of caliber, bullet construction, etc. offer very small performance effects.
The only old school lessons we can keep are, I think:
- The handgun is what you use to fight your way back to the rifle you shouldn't have put down in the first place
- Bring friends with guns
- Only hits count
- Shot placement matters more than what you are shooting
- Under pressure, you don't rise to the occasion, you sink to the level of your training
- Only perfect practice makes perfect
Shoot the cheapest moderately-effective cartridge you can, to get the trigger time and get better at hits and placement (and speed...)
Carry the most gun you can, with the broadest possible definition of most. Data indicates that means more of any moderately-effective bullets. Even well trained good guys miss under pressure, and enemies move more than cardboard, so hits and shot placement become tricky in the real world. I am happy to say 17 9mm will always be better than 8 .45s.
(ETA: If your limit is 10... things get trickier. You need to find if you shoot 9, or 40, better enough to matter, or if practice on your carry platform is more important. Me, I am not at ALL sure that 8 .45s is better than 10 9mm for everyone)