I think this has been discussed several times before, and if those are the only 2 kingdom cards to consider, the best play is 1 Jack, 2 Witches.
The generalization of this is the right number of Jacks (assuming you want one at all) is almost always 1 (the exception is when there is no other terminal you might want), and if there is nothing else to do with your $5, it's right to get a second Witch.