700, it isn't as top heavy as the 660. has a better gear ratio. has better suspension.
i rode a buddy's 660 a couple time. it really sucked in the trails. 1st gear was too tall. if you go to slow and it dies, it will stop in its tracks and you might go over the bars.
test drove a 700 and it was way better. but both these quads are for bigger heavier riders. like people in the 200+ range. if you are lighter, and under 6', these quads will be a handle full because of the size and the suspension will be stiff for you.