Canon 100D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 5 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 100D'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 100D and SL2 sensor size comparison.
Canon 100D 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 100D 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Canon 100D and Canon SL2 sensors have been tested by DxO and the results show that SL2 has a better overall score of 79, 16 points higher compared to 100D's score of 63.