I own a 300 f/2.8 and a 400 f/2.8. I've shot with both the 500 and 600 f/4s as well. I've never noticed any difference in sharpness between the four lenses. The 500 f/4 isn't as heavy as you might expect either.
The 300 suited me better as an every day lens--it comes along in my bag all the time. The 500 is more of a specialty lens. If you're only shooting planes and need the reach, get the 500. You can stick a 1.4 on it and get a fairly decent looking image at 700mm. If you're going to be shooting other stuff and need some portability, the 300 is an excellent choice.
It also depends on the camera you're shooting with. Both of them on a 5D MK
III or 1DX will look really good. If you're using a MK IV
, there might be some ghosting evident in bright sunlight with the 300 and 1.4x converter. It's an issue I've had recently with both the 300 and 400, and it really ruins an image. I'll be happy to email you some files, just PM
You can't go wrong with either one. If money wasn't an object, I'd go for a 300 f/2.8 and a 600 f/4.