Hi Harry
I had a similar query and sought advice from a Canon users forum
I got some good feedback and after accepting the 100-400 was better, but nearly 3 times the cost of the 120-400 and double the 150-500 I brought the 120-400
Most of the replies I got were from bird fans, with them showing the relative merits of whichever lens they had
After a week or so, I decided I was more impressed with 120-400, it was lighter, seemed to have more positive reviews and less negative reviews than the 150-500, and I thought the sample images I was shown were generally nicer
No one had any experience using it for aviation, but a couple had used it for motorsport and showed some of their results
I understand there are 2 versions, an older one without
OS and a newer one with, which is what I got
I haven't had a chance to try it out yet as the weather has been diabolical when I'm not at work, but a few quick tests around home show it has potential (i have been able to handhold at 400 down to quite low shutter speed and get acceptable shots (1/100 at 400 and 1/25 at 120) - they won't win any prizes, but...)
Not sure if this helps or not
Paul