Actually, like the .44 magnum, the .45 long colt has mediocore stopping power on humans.
And yet, "stopping power" is a myth. The only realities are penetration and expansion.
Once a bullet of a specific construction and weight leaves a gun at a set velocity, it pretty much "forgets" what cartridge it was fired from.
You can build .45 Long Colts that are poor "man stoppers" because they use bullets that are not optimized for the velocity at which they will hit the target. Or you can build a .380 or 9mm that is a "great" "man stopper" because the bullet and powder charge used balance the velocity vs. expansion equation well and the bullet works like it should.
A very heavily constructed JHP might not expand at all at normal service round velocities and so behaves more like a FMJ, needling on through. A bare lead bullet might not hold together at higher velocities, thus blowing up without penetrating far enough (or breaking into several pieces each of which creates a wound channel -- which might work well). When these things are balanced correctly the bullet will expand to its maximum without breaking apart so it retains enough weight to penetrate.
All things being equalized (meaning that each bullet is being driven to the velocity at which it works as intended) a bigger bullet going faster is going to do the job more effectively than a smaller bullet going slower. So, a .45 Colt optimized for self-defense, will make a more effective round than a .357 Mag. optimized for self-defense. The .45 Colt has the misfortune that the vast majority of ammo available is low-powered, round-nosed stuff that won't perform well (or as well as the cartridge COULD anyway) on any living target. The .357 has the benefit of being pretty well engineered on the "man stopping" front for decades now and there are lots of effective loads available for it.
To boil it down further, a cartridge that causes a bullet to expands to 1" diameter and penetrate 20" is a better performer than one that causes a bullet to expand to 0.5" and penetrate 20" (or 10", of course). So for a comparison like this to be meaningful at all, much more information must be nailed down about the rounds being compared.
It would be interesting to do some very controlled experiments comparing the very best built (optimized might be the right word) defensive ammunition available for each of the popular cartridges.
Hmmm...we're gonna need a lot of goats.
-Sam