I'm new to these boards but a student of Ballistics.
From a purely scientific standpoint, over penetration is technically ballistic failure of a bullet to transfer all of its kinetic energy into its intended target. For maximum effectiveness, you really want whatever bullet to stop in its target, thus transferring all of its energy into, in this case, soft tissue. If we're talking about larger bore rounds, the energy transference and temporary cavitation can actually liquefy internal organs in a (usually small) radius around the permanent wound channel.
From my studies (which, admittedly, are still in the beginning stages, but I am working toward a degree in forensic ballistics and toolmark examination), I'd personally say that .45 ACP is my round of choice. As far as internal ballistic pressure, size and speed are concerned, a .45 is a big-ass bullet moving relatively slowly (800-1100 fps on average), while 10mm is a smaller object moving considerably faster (around 1400 fps on average) and even though it takes more to stop a larger object, the PSI upon terminal impact is roughly (again, on average) 300-400 PSI higher in 10mm and the rounds have a tendency to punch on through while the .45, with its larger surface area, is slowed in soft tissue more readily.
Take this, and add the lateral stress from expansion of the JHP ammo that most shooters use for defense, and you'll see the .45 bullets either stopping or bouncing around inside the center mass as opposed to the 10mm which creates a smaller wound channel, exits the target and embeds itself into a wall behind the BG.
However, despite all the numbers, math and speculation...if we're talking about raw stopping power, a hollow point is a hollow point and shot placement is key. You can drop a perp just as fast with a .22 as you can a .500S&W if the bullet is in the right place. The question is simply this: how big and bloody would you like the hole to be?
On this, and I am in no way trying to start a .45 ACP vs 10mm debate here, it displays the same issue I have with .40 S&W. It's a great round, but the recoil is sharp as hell. The 10mm has an even sharper recoil, thus making it not the right round for me personally (.45 ACP has a softer recoil and is easier for me to control). Some people like Fords and some People like Toyotas and even others like Cadillacs. It's all about what you, as a shooter, can control the best and hit with most accurately. Again, it's all about shot placement. I just happen to be able to place my shots better with a .45 than a 10mm.