Should I choose MDF or plywood for my false stair treads?
If it is MDF or HDF (as opposed to particleboard) then I would choose the fiberboard over ply. 

Any surface wear will impact the finish laminate, regardless of the underlying material.
By contrast, MDF/HDF should be more impact resistance, water/mold resistant and uniform in density and composition.



MDF above: more uniform and dense. Particle below, less uniform and dense.