FujiFilm F70EXR has a 10.0MP 1/2-inch (6.4 x 4.8 mm ) sized CCD sensor and features EXR processor. On the other hand, Fujifilm F800EXR has a 16.0MP 1/2-inch (6.4 x 4.8 mm ) sized EXRCMOS sensor and features EXR processor.
Fujifilm F800EXR's sensor provides 6MP more than FujiFilm F70EXR'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 F70EXR and F800EXR sensor size comparison.
FujiFilm F70EXR and Fujifilm F800EXR 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 FujiFilm F70EXR has 62% larger pixel area compared to Fujifilm F800EXR, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.