Do you shoot large bursts? Your question was based more on focal length of lens, not on speed of camera. Based on specifications, including burst (20 fps), AF coverage, and buffer capacity (more due to smaller image size), I believe that the A9 might be better suited for sports/action photography, specifically aviation. Sony designed the A9 for the action photographer, while the A7RIII is geared more toward landscape shooters. Granted, any of the cameras can be used for any purpose, but the specs are such that each has a better design for certain situations. At 24mp, the A9 is not going to be great for cropping though. My plan for the next airshow here is to rent the A9 and use my Sony 100-400 with the 1.4x teleconverter. That puts me at effectively 600mm at the long end.