In this instance and I haven't got a D600, but a D800 which I upgraded from a D700, I would recommend at the price you've been offered the D700. The caveat here is what you need or want the camera for. If you shoot landscape, portrait etc the D700, but if you shoot eg. wildlife where perhaps tight cropping occurs, you may be better off with the D600 purely for file size.
The choice is yours, but the D700 is a legend of a camera and of course the build quality is excellent. Plus, as you are getting the grip thrown in it's fantastic value.