In all seriousness, this one is not even close. Thea's only redeeming quality is that she is able to bear a son that has healing techniques. Sari is incredibly strong, even brokenly so.
Yeah, Sari's strength is very useful. Thea is a good healer, but so are a lot of other characters. And offensive Techniques aren't worth much in PSIII unless your name is Lyle.