Title pretty much says it all. I have the 70-200 f/4L IS. It felt plenty long enough when I had a crop body, but it is simply too short on my 5D. We have a pond in back of our house, and I like to shoot the ducks and birds that hang out there. But 200mm is not long enough. I am considering getting a 1.4x TC, but it seems like I could probably do a trade of the 70-200 + cash for the 100-400 and come out roughly the same out-of-pocket, but with a substantially longer focal length (400mm vs 280mm).
I do want to get another crop body eventually, but that will have to wait. I do not think I would mind the push-pull design of the 100-400. Are there any other things I should consider? It seems like the image quality should be pretty similar between the 70-200 + 1.4x combo and the 100-400.