If I had the money to upgrade my 50D this would be a real dilemma for me. I would really like to use a 1.4× converter with my 150-600 C, and that really says 80D for the better ƒ/8 AF with off center AF points. I have never really found the need for really high speed bursts when shooting airshows. I usually have the 50D on the slower of the two burst rate options. About the only time I switch to high speed is for the crossing passes of the Red Arrows Synchro pair. The extra four megapixels would also be nice too.
My problem is that I use several of the hardware options that the 7DII retains from the 50D, but are missing on the 80D. I really like the joystick for selecting the AF points directly, and I use the PC socket a lot, thanks to needing studio flash units in an area with a lot of RF interference. I also have problems with killing SD cards while handling them, which is another thing that worries me about the newer xxD bodies. I think for me the ergonomics of the user interface, and other hardware differences would outweigh the ƒ/8 AF and resolution, and I would pick the 7DII.
I have noticed several comments that have suggested that a 7DIII may be on the horizon, and right now I might be inclined to wait and see if April brings any announcement. I know it would be better to buy now, well in advance of the new show season, but maybe waiting a month just in case the new camera drops. I don't see Canon leaving the 7DII quite as long as the original before updating it.
When it comes to slow shutter speeds for prop aircraft It's not just to get a blurred background at takeoff and/or landing. A full disk even against a pain sky always looks good. With a three bladed pro you are going to want to be at or slower than 1/60 for a full disk. A two blader will need even longer, I would be looking at slower than 1/30 for that. Those speeds would be for aircraft at more than half throttle on take off, or a fast pass.
These were shot at 1/80s and have also had a 1.5× crop applied. Which I find I have to use quite a bit, and hence why I would like to be able to use a 1.4× converter.
I use a shoulder stock to help support the camera/lens combination, and it also helps quite a bit in keeping the swing of the pan smooth. I am managing far more keepers since I got it sorted out the way I like it, combined with the 150-600.