Nikon P900 has a 16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor and features Expeed C2 processor. On the other hand, Nikon DL24-500 has a 21.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Expeed 6A processor. Nikon DL24-500's sensor provides 5MP more than Nikon P900'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 P900 and DL24-500 sensor size comparison.
As seen above, Nikon DL24-500 has a 4.1x Larger sensor area than Nikon P900. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.