Why a spotting scope for aircraft tracking? A pair of bins would be better as two eyes are always better than one. A spotting scope is ok I suppose for writing down aircraft numbers when planes are stationary on the ground.
As with most things in life, you get what you pay for. Avoid some of the fancy colourings on certain makes of cheaper lenses. Most importantly, buy a pair that has quality BAK 4, (as opposed to cheaper BAK 7), lenses and make sure the optics are FULLY MULTI COATED as opposed to just MULTI COATED!
Buying binoculars is a very personal choice when it comes to weight, feel, build quality, etc.