Considering that this is a cycling section I think we are all pretty partial to mountain bikes so thats what I would get.
but get a Dirt bike. Yamaha's are really nice.
i got atv and motocross bike. i would go with the motor bike because it hurts less when you come off. i should know. but if u had a 660 yammie you may as well get a better quad because you seem like ur pretty good to controll that beast