Here is a gallery of 100% crops from the 400/5.6L. Almost all wide open. The AF on this lens is lightning fast, it is lightweight and is about 1/3 stop brighter wide open than the 100-400. The 100-400 is actually about 385-390mm at closer range while the prime is 400mm. The 100-400 can focus at a closer distance but is offset a bit by its actual focal length < 400mm.
I have owned both and the only thing I missed about the 100-400 is its zoom capability. I think you would be happy with either and I would weigt it as zoom+IS vs speed and weight, though the nonavailability of the IS would not put me off.
View these at their \"original\" size. exif intact:
