It depends. Mainly on who the writers want to win, but also on the circumstances.
Scarlet Witch is much more powerful, but Zatanna is more skilled, and frankly, is a better fighter. She is more used to operating on her own under tough conditions. If she has the chance to cast a decent spell, she could end SW quickly, knocking her out or killing her. However, SW can warp reality to a degree, and she gets more powerful the more out of control she is.
Thought that Zatanna was a high end magic user in own right though?