1canvas, it's more a matter of tailoring the tool to the task. In cold climates, where an opponent is likely to be heavily clad, I've seen enough bullets fail to expand and turn into ball that I want it to turn into BIG ball. So, in cold weather, I generally carry a .45. In fact, I'm ALWAYS comfortable carrying a .45.
I'm comfortable with a handful of the best 9mm loads in warm weather when an opponent will predictably be lightly clad. I often travel with a 9mm because the airline weight limits allow me to have more of my chosen rounds with me, and practice/training/match ammo in 9mm is cheap to replenish and easy to find on the road. Trouble with 9mm is that there are only a few loads I'm personally comfortable with, and we've all seen the ammo droughts of Y2K and the post-Obama election where our preferred ammo simply wasn't in stock. The larger calibers are simply more forgiving of less than optimal ammo choices.
And the .40 is certainly a proven middle ground between the two. Basically, I try not to obsess on caliber. The operative term in "gunfight" is "fight," not "gun."