Edink says the 9mm does not deliver the same stopping power as the .45 acp cartridge. I think it's got stopping power close enough to the .45 and sometimes better with the right load. And it's a lot easier to shoot accurately.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master