Buy a D600 or a D700 ?

That's what I did.

Like you, I wanted to go to FF from DX (with my beloved but nearly obsolete D200).

I compared the D600 and D800 side by side at my local dealer. Dust problems aside, I found the D600's build quality and feel somewhat amateurish. Plus, I didn't like the ergonomics/button layout.

The D800 build quality and feel were terrific, but I had no need for 36 megapixel images.

I bought a used D700, in great condition with 15k shutter actuations. Couldn't be happier with my purchase.