Would your hypothetical purchase be strictly for astro use, or daytime as well?
The others have pointed out the many strengths of the 100-400... I guess I'll play devil's advocate and point out a few potential weaknesses
I have a bunch of Canon zooms and primes (from 14 mm to 200 mm), but I haven't used the 100-400 or 400/5.6 specifically. I have noticed that the zooms fare a fair bit worse than the primes in terms of image quality (sharpness, chromatic aberrations, flare/ghosting) on stars - but it's nowhere near as noticeable for terrestrial work.
A quick look at The-Digital-Picture.com's database shows that the 100-400 is quite impressive wide open in the centre at 400 mm, but the corners are pretty average (as expected) on a full-frame camera. Probably not a concern if you're on a crop camera.
http://www.the-digital-picture.com/R...mp=7&APIComp=0
Here's the flare comparison. Again, it's more of an issue for full frame than crop bodies.
http://www.the-digital-picture.com/R...mp=7&APIComp=1
IS is super useful on telephoto lenses. The 100-400 is a relatively old model (1998), so the IS is an early generation - only 2 stops worth of compensation (the newest lenses are 4 or more), and no panning mode. Not a deal breaker at all, but it's worth being aware of.
Have you considered the new 70-300L?
If it were me, I'd probably choose the 100-400 for its versatility and IS if it wasn't primarily going to be used for astro work.