Concrete Backer Board vs Plywood for Bathroom sub floor

Bathroom floor has dipped. Think Structure is concrete, with the subfloor built using chipboard on top of joists. (Flats built 2005). Whats the best choice for bathroom floor to be most waterproof? Concrete backer board OR Plywood? We would be putting sheet vinyl on top unless advised that tile would be a better option or some reason. And would this require removing toilet, sink, bath and boxing?