I am not sure I am entirely qualified to answer your questions but I will try to help. Your question of efficiency (aka endurance) seems to indicate you are looking for lowest cost per mile. If that is correct I would suggest a gas engine because the cost for a gallon of fuel is 1/4 to 1/5 as expensive as glow fuel. If you are considering a twin engine aircraft I would suggest a design that is a tandem (push/pull) type aircraft. If you lose an engine on a twin however, your endurance will be greatly affected by having to prevent a yaw condition if you can continue to fly at all. A tandem setup will simply result in deminished power but you won't have to fight a yaw effect.

If you are considering a twin because of reliability I would go with a single engine setup. Most of the gas engines are extremely reliable when adjusted properly. They are also very efficient. I recently helped a friend setup his Hanger 9 Piper Cub (100" wingspan) with a Zenoah gas engine. I think he flew that plane close to 45 minutes at 1/4 throttle on a single tank of gas. If you build light (use light wieght materials in your design) you should be able to construct a high lift wing structure that will give you substantial endurance at a relatively low cost per mile flown.

Hope this helps a little!