Canon T6 has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4+ processor. On the other hand, Canon SL2 has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 7 processor. Canon SL2's sensor provides 6MP more than Canon T6'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 T6 and SL2 sensor size comparison.
Canon T6 and Canon SL2 sensors have exactly the same size so none of them has any advantage over other in this area.