Do aircraft obtain better milage by flying high or by using "ground effect?"
There really isn’t any strict single rule on whether an aircraft should fly high or use ground effect to increase endurance. It all depends on the design requirements and the purpose of the aircraft.
For cargo type aircrafts, it would make sense to use ground effect and fly over the sea, since speed is not really a major concern.
For reconnaissance aircraft, using ground effect is obviously impossible. For that they have to fly high and the optimum altitude would depend on aircraft parameters, and be a strong function of weight and possible induced drag.