You will love the 1521/1Y on 6S. If you choose to go with a higher voltage go with the 1521/1.5Y for 10S or the 1521/2Y for 12S. Make sure you gear your truck for the speed you desire, but around 40mph is plenty fast for any big truck. Here is a little video
http://www.youtube.com/watch?v=EFoYkI_-v_M it gets good at 2:30. Make sure you get a top notch lipos.