The pattern is on the S model they fix or add something but they don't change the sensor out. So it might be the D5 before we see a sensor change.
It's just like the guys wanting the D600 sensor in a D800 body already!
I don't know this for sure but at least so far this iteration of FX, Nikon is asking us to choose a sensor and handling package at the same time.
It is an interesting dilemma. I don't know how much or early Nikon is going to have to iterate. Myself I would prefer a D800ES with 6 or 8 fps so I waffle on getting a D800E. Not sure if that is going to happen either. I do know that getting 4 or 5 years out of my camera purchase is pretty helpful. Could you live with either the current D600 or D4 for 4 or 5 years? That might be a helpful way of deciding. I guess D600 is so cheap you could amortize it quicker (whether you are a pro or amateur) as a coping strategy.