If you take the used D700, you'll get an excelent camera, same AF as the D300 (i guess), better low light high ISO, better DOF, same controls, same grip and same battery type as your D300. The D600 route means new technology, new sensor, more pixels, not so big camera, not so good AF system (i guess), slower fps rate, new grip, new battery.
I was "almost" in the same boat a few months ago, coming from a D90, and, after the D600 was announced, i opted for the used D700. For me, it has better button layout, better built camera, share the same battery as my D90, better AF than the D600, enough pixels.
Nikonians!!! My best investment made after my camera!!!