There's the story of the Texas Ranger who, when asked why he carried a .45 said, "Because they don't make a .46!"
Rob Leatham, USPSA superstar, carries a .380 I've read, but then he can shoot the eyes out of a gnat in record time, on demand.
I carry, and compete USPSA with, a 9mm. Ammo's less expensive, and the premium defense rounds will do the job if I do mine, I believe. I do carry a .45 sometimes for the same reason the Ranger did...big hole. FWIW.