Between the 06 and 07 660, there is nothing different.
But the 700 is a whole different machine, probably a different class of 4x4. It's the next best thing.
But the extra money, that is something you'd have to decide, if you got the money and it wont put you in a pinch. WHY not just get the 700. But the 660 is one hell of a machine in it's own right, it wont do you wrong with what you want to do.