Axel, I have the D700 and looked at a D600. Neat camera.
Questions: What does the D700 NOT do well for you that the D600 might/will do well for you? Are you okay with the body/control layout? Are you okay with the build quality?
We are talking a generation of maturity here> D3/D700 = Pro/semi-pro while the D4/D800/D600 is Pro/Pro Semi-pro/amatuer, entry. But the D600 build quality isn't that bad now is it? How rough are you with equipment?
Personally, I'd put the money of a D600 towards better lenses, but that's me.