Nikon D7100 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor . On the other hand, Canon 70D has a 20.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor and features Digic 5+ processor.
Another difference between these two cameras is that Nikon D7100'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 D7100 and 70D sensor size comparison.
Nikon D7100 and Canon 70D have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
Both Nikon D7100 and Canon 70D sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, D7100 has scored 83, 15 points higher than 70D.