Any two larger gears meshed together are more efficient then small ones. A larger gear will weigh more but we are not talking much in the rc world. A larger spur is easier to climb. Ask any real drag racer, he will ask for a specific gear with the LARGEST ring or spur, because it is easier on the motor or power plant. So I would say go with as large a spur as you could run, with maintaining the gear ratio you need.