As 1e gives us the only real means of comparison, let's use that:
Demogorgon and Asmodeus are Lesser Gods (DDG p.90).
Armour Class: Asmodeus has a -7 AC vs. Demogorgon's -8.
Hit Points: Asmodeus has 199 hp vs. Demogorgon's 200.
Physical Attacks: Asmodeus has one attack for 4-14 damage vs.
Demogorgon's 3 attacks for 1-6 for each tentacle while his tail drains 1d4 levels.
Magic Resistance: Asmodeus has 90% vs. Demogorgon's 95%.
Special Mentions:
Asmodeus can fly, is as strong as a storm giant, and can use the following abilities; wall of fire, ice storm, wall of ice, detect invisible, invisibility, dispel magic, hold monster, shape change, beguile, rulership, geas, symbols of pain, insanity, and hopelessness, unholy word, summon 2 greater devils (we'll assume pit fiends). His gaze causes fear, weakness, or chill. Also his rod has the effects of dragon's breath.
Demogorgon can hypnotize, beguile, insanity, detect invisible, dispel magic, polymorph self, wall of ice, telekinese, feeblemind, project image, power word stun, any symbol, gating in a random demon.
Too close, it would depend on if they were played to their fullest potential.
Asmodeus = 70,965*
Demogorgon = 74,000*
*material forms only, otherwise x10
But really, with power levels this close any one battle is going to come down to the rolling of the dice. Demogorgon will still win the majority of the time though.
The notion of "material forms" is in 1st ed AD&D - a Demon Prince/Archdevil killed other than on its home plane is only banished, not permanently destroyed - the phrase "material form" seems to have the meaning "form on the prime material plane" - an outerplanar creature's analogue to the bodies that astrally projecting characters make when they arrive on the outer planes.BTW they all have only material forms. Avatars didn't come in to use until the 2nd edt of AD&D
Was it ever stated in either the MM or DDG what level/HD either ruler were? I thought that the MM stated that they were 10th level?
Is this a thread necromancy record? It's gotta be at least close...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.