I'm mostly going on what reading I've done when trying to choose myself.

So my guess on your bucket demolition experience is that it is because of the velocity of the 9mm. 9mm is pretty high velocity and will tend to go through things, perhaps, rather than get stopped and deliver all its energy in one place. I remember watching a video where a 9mm round would consistently go through a frying pan but a 45 just made big dents. So if you're shooting 9mm FMJ practice rounds they might do less damage because they continue on through. Round choice seems to be important for defense. You'd want to use hollow point rounds.

I do know that perceptions are changing and lots of people in law enforcement seem to be switching to 9mm. One of the issues, also discussed in another article someone posted here, is recoil. You can get a 9mm back on target and fire again quicker than you can with a 45.