The .38 Super was commonly carried by law enforcement officers in the USA during the Capone era because it was the best round for penetrating gangster body armour. It remains one of the best (non-special) rounds for penetration.
A comparison of lethality vs stopping power has been given by looking at a hypersonic darning needle vs a thrown half-brick. It's possible to move the needle fast enough that is has the same kinetic energy as the brick. The needle, with proper placement, has high lethality, but an adrenalin-soaked target might not even feel it until he bled out inside. The brick has great stopping power but, unless you get hit in the head, has low lethality.
Moreover, as mentioned, early 20th century US Army trials in cattle showed that it took many FMJ .30 Luger bullets to kill the animals vice just a couple of rounds from, eg, .45 Colt. This was despite the .30 Luger not exiting (thus having shed 100% of its KE) and despite the .30 Luger having a far higher muzzle energy.
In short, KE is nice, but in pistols it does not seem to be The prime factor in stopping power.
Bottom line is that large diameter bullets - for whatever reason - cause more immediate distress (stopping power, ie) than smaller ones. Bullet diameter seems to trump high velocity. Heavier bullets help and Keith-style bullets work better than round-nose ones, which in turn work better than pointed ones, all other things being equal.
Cards read.
Of course, as noted elsewhere, the Ace is always placement. Too much gun makes that harder, making it less effective overall.