I can only comment on airshows and I have no experience of third party lenses. I spend two years using Nikon 80-400VR and Fujifilm S2-pro combination. I found most flight shots at 300-400mm due to CAA regulations restricting proximity of fast jets to any crowdline. Taxy shots 80mm was a little too narrow and I sometimes used a 24-120VR for this purpose. Whilst 100-300 would fit the wide range of focal lengths required it does depend what you want of an airshow. I found VR an excellent aid to prop approach shots allowing lower shutter speeds capturing prop blur yet keeping fuselage nice and sharp.
If your budget is fixed at £500 or less then I would go with 50-500 for its range and its a popular choice along the crowd line, I would guess 30-40% of all SLR owners use this lens along the crowd line. However there is no VR for those prop shots and maximum aperture at 500mm is f6.3 and I assuming the lens will be coupled with S3-pro I am not sure of autofocus accuracy above f5.6.
My advice:- try 50-500 from a used source then save for a 80-400VR from Nikon or Sigma. If 50-500 does not workout then sell on used market and you should not loose too much as they are popular and in demand. A used copy of 80-400VR should sell for £750-800. Using a monopod may be an answer to the lack of VR but they are difficult to use when panning is required through 180 degrees.
I currently use 200-400VR f4 and monopod or 70-200VR and TC17E combination if crowded.
Sample prop photo.
http://www.worldphotographyforum.com...500&ppuser=101