If you look at the book street stoppers.
The guy who wrote the book took real world shooting that were hits in the chest and figured out what each brand and design of bullet actually did to stop the fight.
9mm ball, 45 ACP ball and 32 ACP silvertips all were in the low 60 % Once you started to get into the soft and hollow point designs the 9 and the 45 still runn neck and neck for bullet design with the 45 being only a few percentages over the 9.
The author looked at how many rounds it took to stop the fight. the statistics may be more in line with the 9mm being tied evenly with the 45 because the 9 is easier to to double tap with and a second shot is thrown quickly while it takes a little more time for the gun to recover from the 45 shot and the effects may already be seen.
I do not think that one is better than the other. Any handgun is a poor self defence weapon when compared to other options like a shotgun. They are just easy to have on or with you. I have 380's, 38's, 357's, 9's, 40, and 45's and depending on what I am wearing and headed as to what I would carry. No matter what, you are going to make a trade off of power and bullet size for capacity or it being able to be with you and still hidden. As well as what is the cost of your practice ammo and how often you get to shoot your gun.
The only thing I can tell you is be confidant in what you carry will work with the ammo you choose to use. Buy the best design bullet you can and in the bullet weight of your practice ammo. Practice often, get involved in some kind of handgun shoot (IDPA) and when you practice, every so often run a mag of your carry ammo thought you gun.
Think about driving. Yes you know how to drive and your pretty good at it and if an emergency pops up while your behind the wheel you can handle it, but that is only because you drive everyday. If you never drive ... You get my point.