I carry a 1911, thus a .45 fan, however, that being said, this is a very old argument. From my standpoint (no pun intended) there will never be an "optimal" situation for either caliber. The news stories you read or head about where police fire umteen rounds, that is their TRAINING. Most local law enforcement are taught and trained to continue to fire until the threat has stopped. On the other hand, the military police and defense forces are trained under "live fire" simulations. The military are also taught shot placement and team work. If you have ever seen a SWAT team work, that is as close to military discipline and team work as a civilian organization comes.
I have talked to many law enforcement officers and solders and the overwhelming consensus is shot placement. In an actual fire-fight, one does not have the time to accurately aim and place each shot. Most peoples REACTIONS are to keep shooting until the gun is empty. I had a fortunate opportunity to participate in a live fire exercise as a civilian. At that time, I usually shot 3-4 times a week, and 200-500 rounds each time, all "tactical" shooting. At that time, I shot with a group of current and former military as well as several law enforcement officers and a couple of FBI agents. The courses that were set up were anything from military style training to the FBI style training. (I don't envy either group!) I was taught to do a double tap, followed by a single tap. So, do the math, out of a possible 9 rounds in my 1911, I could only take down 3 bad guys before I would have to do a magazine swap. Most of the courses, required taking down anywhere between 3 and 12 bad guys. Anyhow, back to the live fire exercise, in the first encounter, I EMPTIED my 1911. 9 rounds, at one bad guy. I hit the bad guy 3 times. In fact, after watching the video, I hit the bad guy with my first 3 rounds, BUT, the bad guy was still discharging his firearm, so I kept shooting, and missing. I was asked how many rounds I thought I fired,; my answer was 5 or 6, not 9. Because of adrenaline, I did not have the situational awareness to check my firearm. The slide was locked back, and the next bad guy my team encountered, I brought the firearm up and pulled, not squeezed the trigger, several times. Obviously, nothing happened, and I was actually shot, 3 times. (That's beside the point here.)
The point in this long winded post is my shot placement on the first bad guy was 2 , center mass, about 3 inches apart, and the third was just below the neck. The guy was down, however, that did not register to me because he was able to discharge his firearm again, which I interpreted that I missed, so I kept shooting.
After talking with the military guys, they said, no matter what gun you carry, the only thing that will save you in a fire-fight is training. So that you act on instinct and not on thought. I was told, "By the time you think you should shoot, it's too late." That part of instinctual shooting is what I strive in my personal training to attain.
So, 9mm vs 45? I choose to carry the firearm I am most comfortable with. I know my 1911 and how it reacts when I pull the trigger. I know pretty much where my shot is going to hit if I am within 15-30 feet of my target.
My personal belief is as far as caliber, it doesn't make a whit of difference, so long as you are confident with your firearm and your ability... The only exception here is something like a .22 or .17 HMR.
Yes, my signature states if you can't hit it in 8, you sure won't hit it in 21, but that is just to tweak the nose of my Glock toting friends (and the funny thing is most of my friends prefer the Glock to the 1911).
And I agree with ClearSiteTactical in that there is no penalty for too much ammunition. I pack my full size Taurus PT-1911 and 2 additional magazines. If I am heading to the "gang banger" side of town for what ever reason, I carry my Galco shoulder rig with two magazines, and a total of 4 additional magazines on my belt. (I don't go there very often though.)
Just keep in mind that this age old argument is primarily a matter of opinions. Training and practice is what will keep you alive.