I wonder why they bothered with the 5ds. Why not just have one model to track? Does the 5d do something the 1D doesn't?
SILENT MODE. It beats the living S___ out of the 1DX for being quiet and that is a very important consideration at this level.
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, MP's or AF, it is about the light.