Considering it's been a few hours, I might as well argue in favor of Slaanesh, surprisingly enough, because of a passive ability. Slaanesh embodies not just lust and excess, but also sensations themselves. This is why even looking at Slaanesh makes it nigh-impossible to fight back, as seeing Slaanesh is not just seeing some random daemon/god, but seeing what you want to see most. Your greatest desires and feelings are reflected in the god, and even the strongest of will won't lift a finger to actually fight back unless there truly is nothing they love more than...well, fighting. Slaanesh does not need to actively perform some sort of conceptual assault on Maria to do this. By its very nature, this is merely a state Slaanesh is constantly in, unless it wishes otherwise.
Maria is still very much dangerous to Slaanesh, what with things like the ability to directly attack concepts and her Red Truths, but it won't actually matter when Maria does not actively wish to fight or even resist Slaanesh. Being an already emotionally unstable child, it's not as if she can simply shrug this off. With Maria not being able to defend or attack to her fullest, this leaves an extreme amount of openings for Slaanesh, who has the potential to override low-godly regen and thus end the battle.
If Maria is not defending on a conceptul level, then it's quite possible that Slaanesh could alter her fate to "Maria is consumed by Slaanesh and has her powers absorbed" (note: this is not the fate manipulation that most gods of Slaanesh's level could use on simple mortals, but the fate manipulation acquired from devouring
Morai-Heg and the Wheel of Destiny, which also controlled the fate of gods and beings on Slaanesh's level), not allowing her the opening to try and deny this outcome.
Because of this, I'm honestly seeing it go to Slaanesh more times than not.
