I am not sure how we can be making low light assumptions about a camera that isn't even shipping yet (Canon 6D). Between a Nikon D600 and a Canon 5DMII, that's a no brainer: Nikon D600. Currently, the D600 sensor is only bested by another Nikon camera (DXO labs). The D600 is more comparable to a 5DMIII. And even then I think the Nikon camera is better. And I would always rather buy newer technology than older technology. The D700 is a great camera. But it is outdated tech compared to the D600. Additionally, you are going to see a large difference in detail between a 12MP camera and a 24MP camera. Right now, Canon has some catching up to do. But this hasn't always been the case, Nikon and Canon tend to "leapfrog" one another in "better" cameras, but right now I think Nikon has leapt over Canon. However, I will also say that the the ability to get the image you want lies more in the skill and knowledge of the user. Keep in mind that you can still use DX lenses on a D600. The camera will show you the "Crop" guides of what the image will be. On a final note, don't think the f/2.8 DX to F/4 FX is really how you want to justify full frame, although I will agree that full frame provides more control over shallow DoF if that is your goal.