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, Kodak Astro Zoom AZ651 has a 21.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor . Kodak Astro Zoom AZ651'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 Astro Zoom AZ651 sensor size comparison.
Nikon P900 and Kodak Astro Zoom AZ651 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.