i like 45acp because i can load it into a carbine, get over 600ft/lbs out of factory ammo and still remain subsonic and suppressible while also being able to make brass out of .308, 30-06, and all their derivatives, plus when you bring 45acp pressures up a bit higher the amount of energy you get out of them blows any other pistol caliber out of the water in terms of energy
what i like about 9mm, very, very inexpensive, easy to stockpile ammo by the thousands, cheap reloading components, and a much greater variety of firearms chambered in this cartridge
why do i mention these added benefits of the two?.. because strictly speaking from a self defense standpoint, 3 rounds in the mag (17 in 9mm, 14 in a truly doublestack 45acp), the energy differences out of a pistol muzzle with factory ammo, none of it in itself is enough to really matter, both will do the same job about as equally as well and since im not one to want to stock up on multiple calibers for the same thing i look to other benefits each one brings as the decision maker