The answer depends on your budget and what subjects you shoot. FX bodies have better high ISO performance, more Dynamic Range, and a narrower DOF than DX bodies. FX lenses are also quite a bit more expensive than DX lenses. If you shoot sports indoors or night games under the lights, the D600 would be the better choice. For just about anything else, both bodies are more than capable of performing extremely well. If you compare and image captured with a D600 to an image captured with a D7100 most of the time it would be extremely difficult to tell the difference between them.