For an airshow, the 200-400mm f/4 is usually the safer single-lens choice because the zoom range gives you much more flexibility as aircraft rapidly change distance and framing. At airshows, even 400mm can sometimes feel short for distant passes, but for takeoff, landing, or closer action, long focal lengths can also be too tight.

The 400mm f/2.8 prime has advantages: faster AF, an extra stop for higher shutter speeds, better sharpness wide open, and it works well with a 2x teleconverter when you need more reach. If your priority is maximum speed and image quality, it’s the stronger optic.

Still, because there are no do-overs and framing changes happen fast, many shooters prefer flexibility. A common approach is a longer setup plus a second body with a shorter zoom for ground shots, takeoff/landing, and wider views.

Also, a monopod may be less helpful than expected for panning in a crowded airshow environment. Being able to rotate with your body and keep the lens mobile can be more important than monopod support.