Between a couple combat tours as an infantryman and 12+ years as a street cop in a major metro city, Ive seen hundreds of gunshot wounds. Ive seen several hundred people shot. Ive treated dozens of gunshot wounds. Ive shot a few people. Some of my observations.
Shot placement and penetration matter. A good hit is a good hit and a bad hit is a bad hit. If you want to stop a determined or drugged up attacker you need to cause massive damage to important organs and/or major structures as fast as possible. Barely nicking a major organ or artery might kill a guy, but it doesn't really matter if he bleeds to death 15 minutes after he's stabbed you to death. Also be ready for the bad guy to have little to no reaction to being shot, even fatally. It happens and it will surprise you. Pick a cartridge that will reliable penetrate well and expand. The FBI testing protocol has shown to mirror real world results. Bullets that pass tend to perform well in real life shootings. Bullets that dont pass tend to have far more shortcomings in real life shootings.
My department switched to 9mm a few years ago. Before that it issued .40 and 9mm or .45 were optional via private purchase. New guys are getting 9mm but they havent replaced any guns already fielded. We have a good mix of 9mm, 40, and 45s on the street. Outside of freak incidents all have performed the same... Good hits are good hits, bad hits are bad hits. The few freak incidents have actually all come from 45s where guys shot with it should have been DRT, but they survived. It has nothing to do with the 45, just sometimes people can be amazingly resilient.
I saw a terrorist get shot by a sniper, 175 grain Sierra Match King, the the side of the lower chest. Ripped the front of his torso wide open right at the bottom of the rib cage. Destroyed both lungs, the liver, and the major blood vessels below the heart. That dude ran like 150 meters, with his intestines starting to come out, before he fell over and died. Had one guy get shot in the lower back by a 240C from an Abrams. 7.62 Nato ball ammo hit him in the lower right back and exited his left nipple. He was crouched running away when he was shot. We found that guy a good 30 minutes after the firefight and he was still alive when he got air evac'd.
I was issued a Glock 22 .40 and switched to a Glock 21 .45 for less recoil and faster follow up. I switched to a 9mm for the same reason. I won top shooter in my Academy class with an overall score of 99.28 percent, so I can shoot a 40 well. I just shoot a 9mm better and when having to use a little pop gun in a firefight ill take better accuracy during rapid fire and more ammo. Ive never said at the end of a gunfight "I wish I brought less ammo".