Fri 01-Feb-13 09:40 AM | edited Fri 01-Feb-13 09:41 AM by Ray B
I agree with the advice you've already been given.
Having both a D7000 and D600 myself another big difference in favour of the latter is high ISO noise (or lack of it). If that's important to you the D600 blows the D7000 out of the water (no pun intended).
Another feature of the D600 that is greatly improved is the focal length aware Auto ISO for shutter speed, I find that very useful!
Really though both are great cameras and it comes down to what you want to shoot and how much you value the extra features and IQ?
As for lenses you are really in for a larger investment moving to full frame and that's something to consider. What the salesman told you is partly true in so much as the very best lenses are often FX and the best lenses will give you better better results on FX and DX bodies, but the D7000 with the very best lens won't compete with the same lens on the D600 at say ISO3200 for example. At base ISO in good light things even out though.