Just a stronger connection to the Force, really. They're about even in terms of raw skill based on showings, though Maul boasts the superior tactical/adaptive ability, but it probably only compensates for Jaina's unnatural form at most. Jaina's also just more powerful and masterful in the Force. Maul's pain tolerance is better for certain, but ultimately Jaina's just more powerful and that's more of an advantage.
The same (or most of it, at least) applies to Starkiller and Malgus.