OKay, Some basics.
Getting shot with anything hurts, and will lead to death in about half the cases unless immediate medical attention is given.
9mm as used in WW2 subguns, was and is clearly different from 9mm loaded into a handgun. The extra few hundred feet of velocity added because of the 8-10 inch barrel, and the added power of a hotter load, equalled a better combo of power vs size than most 9mm SAAMI loads today.
Americans learned to fight differently than europeans, and were fairly confident that at the right time, a proper fighting pistol was a good tool to have. European Armies only carried pistols for two reasons, one was a mark of rank, and second, it was the tool to shoot deserters and prisoners.
Americans took that proper fighting pistol to war, and along the way, decided that a weapon that fired the same useful cartridge could be used to break up resistance inside a house or trench quite easily.
Heinz Guderain was not looking at trench warfare when he decided that the new war in europe would not be allowed to resort to the trenches. He was the innovator of the lightning war, or Blitzkrieg, where speed and shock were to be the tools of overcoming the other armies of europe. He wanted supply lines to be simple, with only a very few forms of ammuntion to be shipped. For the most part, he wanted only 9mm and 8mm ammo produced for the troops (obviously here, we are skipping 20mm and up tank, AA and arty stuff) He also decided that equipping large number of troops with a weapon that was made from stampings, that cost something like 21 dollars to produce, and afforded a squad with (almost) the same firepower of a GPMG, was superior to a weapon that took skilled machinists, heavy tooling, and was slow to fire and reload, at least in the warfare that he was envisioning.
For much of europe in the period from sept 1939 thru about 1944, the real image of a boogeyman was a german Para holding a MP40. Soldiers falling from the sky, blazing away with machine guns gave most of europe the runs.
Now the americans show up late for the party, but operating under the guidelines that we will win this war using our rules, and that is saving our troops as best we can, and making decisions based on saving the most lives we can of our side, while costing the enemy the most casualties as possible.
We taught our soldiers to be marksmen, we demanded that we have rifles capable of putting the bullet where it was meant to go, and we brought weapons that ended fights when we hit you with it.
Now moving forward to today, Even with fancy ammo, in much of the country, during much of the year getting ammo to expand and do its job is difficult when the people are bundled up in heavy clothing.
So one reverts to the old proven technology, 9mm and 38 sized bullets tend not to deliver that important "fights over" message to drugged up people the way a .45 does. IF the bullet does expand, 60% expansion on a nine is somewhere around .56 cal. so compared to an unexpanded .45, one can see that even with ball, you don't loose much to an expanded 9, and if the 45 only expands 30%, you're out to .58 cal.
Some see this as reason enough to choose a 9.
I hear some argue that they can carry 19 rounds or so of 9 mm, and thus in a gun fight, they can simply poke enough holes in the target to get lucky and hit something vital.
I hear Civil trial lawyers having wet dreams over that kind of attitude. as 19 rounds leaving your gun each and every one have a lawyers business card attached to them saying "Sue me for wrongful death and injury" as the misses carry on down the street and hit that kid doing homework in the livingroom two blocks away.
I want very few shots, all accurately placed, all stopping within the body of the felon who has threatened to kill me. I want no flyers or strays rumbling down the street killing or maiming innocents.
Now, if this argument is simply about subguns, then today our FBI has the best option, which is the MP-10, but for some reason. They forget to teach the Second Amendment at the FBI school in Quantico, because if they did, the FBI would do everything in its power to get that weapon out to law abiding Citizens as the near perfect home defense and civil defense weapon.