I'd say the Jed'aii. The Nightsisters don't have full control of their planet (contested with the light side Witches), and while they have a few *very* powerful elders, the normal Jed'aii seemed more impressive.
Due to their larger organization, Jed'aii have advantages like large councils of seers working together... oh, and while both sides use rancors, their rancors can fly.
The Je'daii have pretty potent battle meditation, whereas the Nightsister's rituals are sure to cause issues. The weather could also be a deciding fact depending on which planet the fight takes place on, because Tython usually goes ballistic if it's Force alignment falls out of balance, leading to devastating storms.