Which is better: a rotor / prop followed by a stator, or contra rotating rotors / props?
Undisturbed inlet flow provides the highest efficiency. 


The single rotor with stator has a straightforward inflow; outflow will have a tangential component which is lost for thrust generation. The stator will bend the outflow back in the axial direction, but there still remain pressure increases with blade passing frequency.
The contra rotating rotor has one of the rotors in the disturbed flow of the first rotor - this second rotor will therefore have lower efficiency in transforming engine power into thrust.