For what it's worth the D600 exceeds the sensor performance of the D700 on every metric according to DXo Mark. But having spent a marginal amount of time looking at the D600 I personally find the ergonomics lacking. The D700 is a more pro-oriented body, has the ability to accelerate to 8fps (if needed) with grip, has faster flash sync speed, has a proper AF-ON button (does the D600 have one?). D600 of course has video (aside from obvinous resolution advantage) if that's of interest. guess it really depends on what's important to you.
Personally I don't think there is a true D700 replacement. It's the last of its kind. You have a choice of double the resolution and consumer body, or triple the resolution in a pro chassis, neither of which with the speed (gripped) of the D700, although both feature updated AF and video among other refinements.