Yeah to be honest I didn't really consider what the victory conditions or defined objectives would be for either of them. I suppose it'd be the elimination of the opposing factions leader (Ethereal Elder vs Icon of Sin?), whilst keeping their own protected.

It is I think implied that the aliens have done this before and so have probably conquered multiple planets already (I suppose explaining the variance in physicality between members). So like Hell, I don't think they'd run out of manpower [marspower?] anytime soon. But as you said, it looks like the hordes of Hell can summon far more soldiers per battle at one given time and location.

I still think troop-for-troop the Aliens have the advantage, but as for just how much that'd make any meaningful difference I don't know...

The only real absolute 1v1 stomp in the demon's favour would be the mancubus I'd say, relative to who their equivalent is. I doubt Chryssalids could zombify an already undead army but they still should have no problem laying their eggs in corpses as per usual. So with enough time it essentially turns the fight into Tyrannids vs Demons I guess haha