Going to attempt a trade from Nikon to Canon this weekend.

The D800 would be the logical upgrade but the D600 seems to offer a better bang for your buck. Your reasons for switching are not worth the $$$$$, considering the cost of obtaining Canon lenses. As mentioned by another poster, the Nikon D2X mode 1 (as well as mode 2 and 3) render very good skin tones as well as WB issues).  The 5D3 has a superior AF system and very fast buffer refresh rate, but that won't give you better photos than your current set up. But, it really depends how and what you shoot.