I used to own a 100-400 but decided to go for a prime lens as the long end I found to soft and I hated the push pull zoom. Don't get me wrong, this is a very good lens and as described above it's versatile with one lens covering a wide focal range. Great at 100-300, soft at the longer range so you need to stop down with the aperture.
I replaced mine with a 300 f/4 L IS
and a 70-200 f2.8 L IS
. The 300 is now my main lens, it is light, fast and sharp. I also use it a lot with the 1.4x extender with great results too. Not so sharp with a 2x though. Fergul was right in that you need the EOS 1 range to get AF
with a 2x extender on an f/4.
The 70-200 is a great lens but with a 2x extender doesn't compare to the 100-400. Close but not good enough to go for that option, I'd recommend the 1.4x for aviation work.
If you want one lens go for the 100-400, but if your budget allows I have been delighted with the 300, 70-200 and 1.4x extender. Maybe also consider the 70-200 f/4 L. Had one of those too, great little lens.