Threads like this tend to get answered in a similar way each time.
* It depends on the author/which faction is the star of the story in question.
* The loser will be the one that's easiest to bring back later in order to avoid permanently killing a character. Daemons are easier to "kill" without permanently removing from the setting, so Lelith has a slight edge. Although drukhkari ressurection tech dulls that edge.
More helpful answer: Lelith should probably win. I know you're asking about lore rather than game mechanics, but stay with me. The stats models are given in the game are
GW's means of telling us something about those models. Back when Weapon Skill was a positive number that got compared, Lelith's
WS was something like 9 or 10 while the Masque's was something like 5 0r 6. Depending on the edition, Lelith also (potentially) had more Attacks
iirc. Invul saves and similar Strength and Toughness values make their weapons kind of a wash. I think the two generally had the same or similar Wounds stats.
So basically, Lelith tended to have much more impressive stats than the Masque. The story these stats tell us is that Lelith is the better combatant. The Masque has their fancy dance magic, but the dance magic is depicted as not being an auto-win for the Masque. So yeah. Seems like
GW thinks Lelith should defeat the Masque more often than not.
Also...
Ranoud 2: 1v1 battle in real space, but the Dark Eldar and demons are fighting around, because of which Lelith constantly becomes stronger due to their suffering and death
While Slaaneshi units don't have a Power From Pain mechanic, you could probably make the argument that they (along with most daemons) would
also gain strength from the warp energy generated by the extreme emotions of those on the battlefield. So pain-vampirism is probably a wash unless we have a reason to think that Lelith gains a significantly bigger boost from suffering than daemons do.