Are you trying to win the all-up-last-down competitions that are sometimes held at electric fly-ins?

My guess is that the 13x8 will prove to be more efficient but only flying will tell. Your goal is to fly at the airplane's minimum sink rate speed (unpowered) with a prop and motor that are both in their efficiency sweet spots. Sometimes that's best done by using the motor's power to gain altitude and then shutting it off and gliding for a while.