A quick thought on metal tanks. Metal is heavy, and hard to work on if you're into body work or mods.
Generally metal chassis are good.. metal lower hulls, suspension, torsion bars (On many Taigen hulls), and sprockets. Metal upper hulls and turrets just add weight and wear and tear, they don't improve the mechanics of the tank.
I recommend the Taigen Tiger or their new Tiger II, excellent runners. If you have the option of a plastic turret, choose that! :P