Just to clarify, my thinking was from the point of view of a wildlife photographer. I now realise I should have made this clear.
I have read the replies with great interest. There is no simple answer, but I suppose it all depends on the lens attached, and getting pixels on the subject. If I was to mount a 800mm lens (I can dream!) on the D600, I could use all 24MP, and get a similar image to a 500mm on the D300 at just 12MP. Obviously, the situation is different if we use the same lens on both cameras. So for me, I can see no justification for buying the D600 over the D800.
I understand there are improvements in dynamic range and ISO performance to take into account with newer bodies.