If you "need" full frame then evaluate the specs of both cameras and decide which bests suits your need.
If you "want" full frame then evaluate the specs of both cameras and decide how much you want to spend on a camera versus lenses or other accessories that you could get for the price difference.
I have a D700 and have no interest in a new D800. I don't want or need 36 MP or slightly improved ISO. Maybe in a few years I'll pick up a used D800 (it is hard to ignore the rave reviews), but for now my D700 does just fine.