The king is back Long Live the King
Hitting what you aim at and being able to do it repeatedly and quickly is primary.
The best SD gun for YOU is the biggest caliber gun that YOU can put 3 shots on target in 3 seconds with. If you can do that with a 9mm but not a 45, then the 9 is a better SD gun & caliber for you. Three center mass hits will almost always trump one hit and multiple misses or on hit and no follow up shots.
Shot placement and bullet weight/type....? fwiw
As many know, my choice is an N frame in 45 ACP. It is what I can handle most comfortably. Are there better choices? Not for me at this time. Ask me when I am 75.
My eldest cousin is currently preferring his 1911s to be 9mm versions. The 45 is too punishing on his arms and wrists. Also, for him the 9’s improve his hits on target because the 1911s apparently recoil less when 9mm.
Practice until you master the .45 ACP (or something bigger, if you wish).
Maybe you will run into a situation where you have only one shot and you'll be glad you had the most destructive round for that one shot.
You might want to be like Ted Nugent and settle for nothing less than the 10mm.
I've noticed that the recoil from the Gen 4 Glocks in .45 ACP is far easier to handle than any other .45 I've used.
The FBI like other federal, military, state, and local agencies buy handguns and ammo primarily on how many new recruits can qualify to be politically correct and then low cost/low bid.
I don't worry much over the 9mm/.45 debate.
Bullet performance has come a very long way since the 1986 Miami FBI shootout.
Put a well performing bullet where it needs to go and all other discussion is moot.