Thing is I'm not even sure about that. Ox (potentially) has the higher ceiling but Navas is very experienced and knows his role. The guy has won quite a few trophies and probably has a few hundred appearances under his belt. He might not have the skills and tricks that Ox has but he won't make the same stupid youthful mistakes that Chambo does.
Plus City also have a few other top quality players both in the starting lineup and on their bench that can make a difference if Navas can't. We don't have that luxury. If Sanchez or Ozil are out then we're fucked.