Another factor not yet mentioned-any need to shoot video? I use all three bodies, 1D-X, 1DIV and 5d3 with the 5d3 being used for video (less aliasing, easier controls).
As for strict comparison, there are actually pro/con for each, but overall user friendly of 1D body is hard to beat. Post -production wise, in my hands, MP/color rendering/DOF of FF 5d is slightly better, but reach/FPS trump those minimal advantages.
When on assignment alone with choice of bodies, I obviously take 1D-X and then the 1DIV....