- EF 400 f/5.6L IS (Highly desired)
- EF 100-400 f/4-5.6L IS (Not push-pull & patents exists)
This will be a tough choice, if both lenses actually appear. I'd expect the prime to have better IQ, but how much better? It's no faster, they'd have the same rated IS system, and the 100-400m range is very convenient.
I really hope the 100-400mm is an extending design - I like the push-pull, but I could live with a rotating extending design. One of the big advantages (for me) of the current 100-400mm is the relatively short retracted size (compared to the 400/5.6).
Given a choice between an internally zooming 100-400 (which would be slightly longer than a 400/5.6 prime) vs. a 400/5.6 IS, I'd definitely take the prime. An extending zoom would make the choice a lot harder - it would come down to the differential in IQ, especially with the 1.4xIII.