The primary reason military tracked vehicles have rear sprocket drive is to protect the engine-tranny from damage from enemy fire. Secondly, with rear sprocket, there is more flexibility of suspension in climbing obstacles. The sprocket in front with no vertical movement, there are limitations in maneuvering over vertical obstacles as well as possible damage to the sprocket teeth. The Israeli Merkhava tanks, completely contrary to basic tank design, were designed as defensive vehicles and have the engine-tranny in front and driver far offset to the side as a protection for the crew, plus an access tunnel at rear center. With building a plastic model RC tank, havng the sprocket near the motor & battery may give some added traction due to the light weight. Consider also the type and width of the track.