What do you like working with? I prefer 4:3 cameras over 3:2 cameras. Big viewfinders are also preferable. Sure, the economics are important, but if you spend so much time with your equipment, how they handle is also a factor.
There is only a difference of 27% between the Nikon and Credo in (pixel/file) resolution--in a linear measurement, which is the right way to measure resolution. So if you are happy with the resolutions of your current system, both cameras are a step up. The sensor size difference will change that in different ways. Noise is very different, but if you always shoot at base ISO, then that is not a factor. Long exposures will go to the D800 as I believe the Credo has a limit to exposure time. Then there is file size--are you prepared in dealing with huge increase in file size, even for the D800. If you go with the D800, you could also dump the Canon and travel with one system rather than two.
I would get your hands on these cameras and shoot them for awhile. There is no right answer here.