Nikon D5 has a 21.0MP Full frame (35.8 x 23.9 mm ) sized CMOS sensor and features Expeed 5 processor. On the other hand, Canon 1D X II has a 20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6+ processor.
Another difference between these two cameras is that Nikon D5's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the D5 and 1D X MII sensor size comparison.
Nikon D5 and Canon 1D X II have almost the same sensor size so none of them has any significant advantage over other in this area.