<<IMHO- as an experienced airshow goer, get the 50-500 OS. It's a Sigma lens.>>
I don't shoot airshows, but the 50-500mm Sigma OS is my first lens of choice for wildlife. The only proviso is that in very low light (as we so often have in the UK) I'll switch to my 300mm f4 or 70-200mm + x1.4TC. Shooting towards the sky doesn't usually prevent the use of the Sigma however.