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.