I am tempted to answer, "the difference is 0.05 inch." But I won't, because that would make me sound like a wise guy.
The .45 ACP is old-fashion, and delivers a relatively heavy, relatively slow-moving bullet. That's not a bad thing. It's just a statement of fact.
The .40 is much newer, and it delivers bullets which, although generally lighter, move somewhat faster.
I like the .45 ACP, and have used a gun in that caliber for most of my pistol-shooting life.
I have never owned a .40, but the FBI thinks that it's the bee's knees.