Hi Colin.
Welcome at the Scramble message board ! It must be confusing to see both Dutch and English messages here, but keep on writing in English and all will understand.
Regarding your question, I don't have a definate answer. But there is a lot to say about long zooms in particular. In the previous two years Dutch spotters have participated in two "Lens-Try-Out-Weekends". This initiative was made to have fellow aviation photographers try out eachothers equipment in order to test if any lens or body would meet their requirements before they buy new gear or have any specific opinion on their own stuff.
The first event resulted in a small personal test of various 400mm (fixed or zoom) lenses. Below you will find the test results in an animated GIF. It clearly shows the difference in quality between types.
Check out
http://www.scramble.nl/forum/viewtopic.php?t=26926 to read more about it, although it's all in Dutch. There's 161 replies in that topic. The first 5 pages are all pre-event while the last 6 pages are all post-event.
I don't know if this answer will help you decide on which lens to choose, but it may put you on a right track.
All the best from Holland,
Iwan Bogels
PS: Keep on participating here, it's highly appreciated !
