Canon 300D has a
6.0MP APS-C (22.7 x 15.1 mm ) sized CMOS sensor . On the other hand, Canon T8i has a
24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor.
Canon T8i's sensor provides 18MP more than Canon 300D'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 300D and T8i sensor size comparison.
Canon 300D and Canon T8i have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.