I once had this debate with one of my old DM's.
He was queer for the Sword of Sharpness and tried to convince me it was far better than a Vorpal Sword. his reason for this was that it hacked off limbs thus increasing your chances of delivering a crippling blow that ended any fight.
But the Vorpal Sword decapitates.
That means the head comes off.
Doesn't that end the fight too?
(OK! So maybe not against a Hydra or even an Ettin.)
And let's look at the way these blades work. (At least according to REAL AD&D.)
The Sword of Sharpness is typically a +3 weapon. (If you look up Excalibur under the Arthurian Mythos in Legends & Lore, it's a +5 Sword of Sharpness so there must be others with different pluses out there.)
Anyway, these bonus points are not tallied all together. In fact you get a +1 to hit, a +1 to damage, and a +1 to determine if you sever an arm, leg, or neck/head.
So if you don't roll a modified score that is high enough to sever, you get +1 to your damage roll and the fight continues.
A Vorpal Sword is also typically a +3 weapon. It's +3 to hit and damage and the +3 to hit roll is used to determine if you decapitate and kill, (In most cases) whatever you are fighting.
I don't have the score charts in front of me so I could be wrong, but if memory serves me, you need an unmodified roll of 18 or more to score a sever with a Sword of Sharpness.
I believe an unmodified roll of 17 is required for a decapitation with the Vorpal Sword.
I understand that it wouldn't be of much use against beasties with no heads.
Slimes, jellies and pudding have no limbs either so the debate here is kinda pointless.
But to me, it seems the Vorpal Blade wins out since you don't need to roll as high and when you do, it ends the fight. Rather decisively I would say.
I suppose if you are fond of gory battles and whittling your opponent down to nothing, then you would have a soft spot for the Sword of Sharpness.
But if you like headhunting.....