I had a similar question a few months ago. I decided to make my own answer by renting a D600 with a good zoom lens and test the results compared to my D7000 with the same lens at comparable focal lengths.
Using a tripod, I shot several scenes with the 24-70 f/2.8 and used both cameras. On the D600 I would set it to 50mm and then set up the D7000 with the lens zoomed to about 35mm to provide nearly equivalent composition.
For me, the difference was obvious and in favour of the D600 in every case. It was easy to pick the D600 shot in side-by-side comparisons on the monitor or with prints. I was convinced and bought the D600.
Some people might not see (or need) the difference.