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Since both the .45ACP and 9x19 are excellent cartridges it is up to the user's judgement with which one to go with. There are differences to be sure, but not enough terminal ballistic difference to the receiver. A well place JHP or SWCLHP in any of the mid bore to large bore cartridges will get the job done under most circumstances.
The only and truly more dangerous exception to the rule has to do with state of mind of an attacker. Fanatical zeal, hard core drugs, or a "man on a mission" mind set are all reasons that pistol and rifle fire, regardless of the caliber,shot placement, will not instantly stop , nor will cause cesstation of hostilities. The Moros of the Phillipines, Zulus of Africa, religious die hards of the Middle East, and the terrorists on a mission of destruction are all examples of people being hit multiple times, even with many non-surviveable wounds and still killed or severely injured scores of others before expiring. One has to keep this in mind when dealing with reality.
In a lot of attacks this mindset may not be present. However with the increase in violent attacks on the rise one has to be ready to deal with this. Shooting once or double tapping an attacker then stopping to re-assess the situation may not work at all. Be ready for multiple shots to the body. Try hitting the head while it is bobbing,weaving, and moving erratically is going to be near impossible in such situtations. One can plan and practice to do this, but the actual real world situation may not allow for it. This is why I will not get into a caliber debate. Even multiple hits with shotgun slugs and buckshot have not stopped certain mindset types.
Police several decades ago shot a guy on PCP several times with full loads of buckshot and slugs that were well placed. This individual also absorbed numerous handgun bullets. A shot to the head finally brought the scene to a close. However the criminal lived for several more moments. He commented the cops should stop shooting him because it burned. Then he expired. Mindset because of the above mentioned factors can act in the same way on the body and make bullet sponges out of even a person with an adrenalin dump. The body starts blocking out pain very fast and as shock may take over. Thought I'd put this up as solid info and for those who think nothing but whatever they carry is the only caliber or bullet that will do.