This lens is small and portable, and it has superb quality even in an astronomical standard at wide open aperture.
In terms of versatility especially when daily photography is concerned, the Canon 100-400mm f4.5-5.6L might have an edge. The quality of the zoom is also very high, but it's not exactly "astronomically" high. Some reported that there could be internal reflection in extreme condition like the diamond ring before/after a totality.
Therefore, if astronomy performance is the prime concern, the prime lens will be better, and the prime lens is also cheaper as well.