I have a D7000 and a D600. The D600 gets used for everything except the bird photos now. The high ISO performance on the D600 is stellar. There is probably at least one extra stop available on the D600.
The dynamic range is also much better than the D7000 which is very very good. My estimate is that there is nearly one additional stop of dynamic range in the D600.
These are just my own estimates, if you check DxO Mark tests you can get the scientific measurement on these.
The best part of the comparison is that the D600 images just seem to have more depth and realism compared to the D7000.
The image files are larger but no problem for my 3-year old intel system with 8GB RAM using Lightroom.