Canon 6D MII has a 26.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features DIGIC 7 processor. On the other hand, Canon 6D has a 20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 5+ processor.
Canon 6D MII's sensor provides 6MP more than Canon 6D'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 6D MII and 6D sensor size comparison.
Canon 6D MII and Canon 6D 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 Canon 6D MII and Canon 6D 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, 6D MII has scored 85, 3 points higher than 6D.