I think the D600 is better than both the D700 and D7000 in almost every way. We're not talking about big differences, but almost all the comparisons favor the D600.
The D600 is just a little bigger than the D7000. It is very similar in terms of layout, controls, and accessories (cables, batteries, cards, etc).
Image quality is a little better with the D600 than D700, but it's not a huge difference in normal use. I find the D600 significantly sharper than my D7000, but that's only visible with a large print or viewed at 100%.
I would probably take the D7000 and one D600 for now. The shared accessories and control layout are nice positives, and it allows you to step up from the D700 to the newest technology with a small cost.