We have sort of discussed related questions under the 'lenses' tag before, but not, I think this specific dilemma.
Just recently (since coming back from France), I've started to get itchy about going longer than the 200mm I've got with the Canon 70-200 f4 L.
It might pass. But until it does ..............
The Canon 100-400 L is getting a bit long in the tooth. The IS has only got 2 stops.
The Canon 400 L f5.6 has no IS. The 400 L f4 and f2.8 (both with IS) are way out of my price range.
The Sigma 100-400 gets very reasonable reviews, but it's not a Canon (so what, it's good?).
Go for the 100-400 L - People still write good reviews.
Sell the f4. Buy the f2.8 II + a 2x extender.
Putting 2x extender on the f4 would make it an f8 (I could go up on my normal 100 ISO to 400, but that would still lose any notion of a narrow DoF). The 1.4 extender would only take me to 280mm (albeit on a crop factor body).
Any beams of light that can show me the road ahead, or even any more thoughts to add to the confusion, would be welcome.
(signed) Confused from Glenfarg