This discussion usually turns into arguments based on one fact or another...
the short answer is, If you can shoot a 9mm into a 2.5" group at 25 yards, and a .45acp into a 5" group at 25 yards, you should likely carry the 9mm
all three will do the job. not one of them comes close to a rifle or shotgun when people start talking about "Stopping power" My friend Jon, who spent 8 years in the Army, another 6 in the guard, and is the son of an Army Ranger would tell you. A Pistol is only useful for fighting your way to a rifle.
That said, Shot placement trumps caliber every day of every week.
+1
Couldn't have said it better