"I know that can't be done definitively" is the operative term in your post, methinks.
FWIW, I think .38 Special and 9mm can be good to go...with the best loads. The smaller the caliber, the more selective the user has to be to get ammo with a proven track record of stopping fights quickly with solid hits. This is a definite concern when ammo is scarce, as it is now.
The more powerful rounds are more forgiving, and there are more ammo choices for them that have good track records.
9mm is absolutely easier to shoot and train with than .45 ACP, and will offer more concealable packages. On the other hand, there are some circumstances under which any hollow point can plug, turning the bullet into flat nose ball. If my bullet is going to turn into ball, I want it to turn into BIG ball.
Best of luck with your choice,