One of my instructors (who is not only a 3rd dan Aikidoka, but a 5th dan Karateka) upon telling him that I was going to buy a gun told me "You need stopping power. You need a .45. You shoot some crazy guy or some guy on drugs, you might kill him but he might kill you before he dies if you shoot him with something less. You need a .45. It will drop any man."
His Aikido is pretty damn good, and I know his Karate is great, so I don't doubt him for a moment. I have always believed that in a world of guns some confrontations can only be solved by guns. Its not pretty, its not harmonious, but it is real.
It costs more to practice than with .40 or 9mm, though.