Been pondering my long tele options over the last few months... A 100-400mm is looking very likely, but I'm left pondering, why would anyone but the 400mm f5.6 prime ?
Serious question, what does it offer other than being very slightly cheaper and lighter ?
The AF on the prime is a lot faster and more accurate. It's lighter and cheaper.
It's IQ is slightly better (assuming you have a good 100-400L) and it's focal length is slightly longer. The 100-400L is closer to a 380mm in reality. The prime offers a slightly better out of focus rendering (the 100-400L can look a but fussy) it also meters closer to f5.6 than the zoom too. It's smaller and has an integrated hood. The 100-400L is quite a long lens when racked out to 400mm and with the hood attached. The zoom is far more versatile, has an Image Stabiliser and closer Min Focus Distance.
For some guys, they buy one or the other. For other guys who have the need for a more capable 400mm lens, then they are not over lapping lenses and often buy both.
Some of us are waiting for the new 100-400IIL to be announced (rumoured but not confirmed) later this year and it could be a game changer and unify the benefits of both these lenses into one unit....or then again it might not!