Upload & Sell: Off
To me the comparison is similar to the D300s to the D7000. With the newer body you lose the more solid build and feel of the old one. You lose the mode button, pc sync and ten pin remote terminals. The older body will probably have a larger buffer (relative to the file size), faster fps with grip, and a better AF system. The newer body should have greater dynamic range, more MP, better high ISO (when downsized to match), a lower base ISO, U1/U2 modes, and better video. The D600 should have a second card slot and be smaller/lighter.
I still prefer the D300s to the D7000, and will probably prefer the D700 to the D600 for the same reasons. One has the technical ability to produce slightly better photographs, but the other (to me) just feels better and is easier to use, and therefore makes me want to take photographs more. I think most people will prefer the newer model though. People used to the build and controls of the professional bodies will prefer the old model.
Edit: I don't think the D700 price will fall too much. I'd guess not more than $200, and certainly not to $1000 for a long time. I also don't think the D600 will be priced under $1800.