Is it more of a comfort issue with having to bend farther down? I would think that riding on the hoods would use more energy than the drops because of the aerodynamic disadvantage (With the exception of climbing hills). I am comfortable with staying in the drops for prolonged periods of time, so should I if want better endurance?
Some riders with limited flexibility can't put out as much power in the drops as they can in a more upright position. If you are comfortable there likely isn't much difference in the power you are putting out but you will go a little faster if you ride in the drops.