Currently just have the D7000, but looking at the D600, for full-frame.
D7000 has very similar features, but being a crop-sensor, it would be like having a 1.5x teleconverter on the D600 for focal length. I plan on keeping the D7000 for telephoto work, as it would have more range without loosing performance.
If I had both, I would think of what shots that would be taken, and then what lenses I would use, then decide on the body. Nice thing with a D600/D7000 selection: both use the same battery (also the D800)!