Canon 90D has a 33.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor. 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.
Canon 90D's sensor provides 13MP more than Canon 70D's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 90D and 70D sensor size comparison.
Canon 90D 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.