Either camera will work for you and will see a significant change in IQ.
The advantage in getting the D700 is that you can use the same battery charger, batteries and battery grip that you use with your D300s. You will have the option to get up to 8 fps with the D800 while maintaining a great 51 pt AF sensors. The price is about average.
D600 offers more MP and probably has better DR than the D700 plus it offers video and other newer features.
I think you mean with the D700, D800 doesn't get 8FPS, regardless.