Canon 70D has a 20.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor and features Digic 5+ processor. On the other hand, Nikon D5500 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor.
Another difference between these two cameras is that Nikon D5500'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 70D and D5500 sensor size comparison.
Canon 70D and Nikon D5500 have almost the same sensor size so none of them has any significant advantage over other in this area.