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 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon T6 has 33% larger pixel area compared to Canon SL2, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.