I wanted a .45 ACP because I do not like 9mm, too dangerous in the fact that it penetrates too much. The 40 S&W I really do not know much about. I felt the .45 was almost as common as the 9mm, but the .45 is big, tough, and American, damnit. And maybe there is no justification for it, but I also do not want any scumbags to be able to shoot back at me. Sort of the same thing that John Dillinger said about choosing a .45:
"I shot a guy with a .380 once, and he shot back."
 DISCLAIMER: I do not want to shoot anyone, scumbag or not. That being said, don't break into my house. A gun lives there, and I just might show it to you.