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.