just upgraded from a d700 to a D800, and at first i had looked at the D600. I didn't go with the D600 cause some of my accessories would not work with it, and the D800 felt like a D700 in my hands for the most part.
At the same time, I still miss the D700. I had a D3 and a D3s as well and I just loved the 12mp low noise files they produced and how fast the cameras responded. But i also wanted HD video capabilities better than the D3s. Ideally the D4 would be my number 1 choice but it was out of my $ range.
My point is that the D600 is probably a great camera but it will also be much different in features and how it feels while shooting and if it's it's just the DXO rating and the fact that it wont cost anything to get id say stick with the D700. D600 is a downgrade from the d700 imo.