[Ronbo]

Plain 'ol stock style chain. Double roller chains (if it's even really a roller link) have very reduced contact area on the gears. Also roller chains have less links. These things cause two problems: The chains with less links are prone to stretch more and the reduced contact area of the sprockets will wear faster.

Gear drives maintain more accurate valve timing (no chain stretch). However gear drives can be more prone to breakage and are noisy. (although the gear drives may have earned the poor reliability rep because of the kind of engines they're typically installed in)