Depends on what your goal is. If you want to tank, of will is better since it has higher armor. The cost of choosing of will over potency is a sacrifice in damage. If your goal is doing more damage, then potency is the right choice for you. I think many would argue (including me) that the warrior is supposed to tank so I would choose will. However, that's just my opinion.

P.s.: this is considering only PvE. If you plan on doing PvP, I can't offer any advice.