Actually the propulsive efficiency of both increases with altitude. The turbine engine is also more efficient at altitude, and the aircraft has less friction and thrust is down it is also easier to push the plane and total propulsive efficiency goes down. With a piston engine the efficiency stays about the same or drops slightly though the prop eficiency goes down the total effect is not that great, thus because the airplane has less friction at altitude it also uses less fuel and greater speed at altitude though not as dramatic as a jet turbine. This is especially true if the engine is supercharged.