There are a lot of sources to use when getting a sense of how one piece of gear compares with another. There are some interesting numbers on cameras/sensors at the DxO site. In their measurements, the Nikon D600 sensor has an edge in most performance areas. Numbers are only part of the story, however. If you have the opportunity, find a Canon D6 and see how it "feels" in the hand, check the menu system, etc. For many, those form factor and usability issues are as important as having a somewhat higher scoring sensor. Neither will make you a better photographer, but the better we can use our tools, the better our output.