Upload & Sell: Off
I think I'd give up my D700 in a heartbeat now for another new D600. That's just me. The D700 is a little sturdier, and I do like the "triple tree" controls. Having said that, I'm hard pressed to find anything else the D700 does better than the little D600. Oh, I like CF cards better... a lot better in fact. Still, single CF vs double SD...
The D600 makes for better IQ in really any situation. I'm pretty comfortable saying that now after my first week or two with it. It's better in good light, it's better in low light, it has more detail, and color rendering is really similar. It has new features, like a slightly improved LCD, HDR built-in, ISO100 native, time-lapse, and if you're into HD video (I'm really not...) well, it has it.
So it comes down to build and controls, and the AF module. Personally I've been impressed with the D600 shooting sports. It tracks very well, acquisition seems to be very good if not QUITE as instant as the D3S I shot it with, and in low light I think it might even be better than the D700.
It's built well as far as I can discern. The D700 is slightly weightier, and side by side you CAN tell it's slightly larger. Not sure if this is a plus or minus for you. The D600 still offers weather sealing and a quality shutter that incidentally is already quieter, but still offers the delay-slap quiet mode option. Most of the controls the D700 offers are still either available outright, or at least assignable or programmable on the D600.
So I'm left with a slightly smaller and lighter body, with most of the controls and build of the D700, updated and added features and options, and better IQ all the way around.
I have loved my D700 (and D3 now sold) since day one. No one argues more than me it was a game changer for Nikon. That sensor was revolutionary not just evolutionary, and put Nikon on top for many pros. But I really have to admit it seems the D600 is now the crown jewel for affordable brilliant IQ.