While I still own some very nice Pentax/Takumar glass I have to admit to being seduced by the Canon range. (...you'll find a number of tests on the web that show the Canon to simply work a little better in the dark).
That said, I have not found a long telephoto lens...by Canon/Nikon/Pentax/Sigma etc.... that outperforms a Taka FSQ (...this is a work in progress, but I promise to put some test results onto a web page soon)
Also when considering a lens for astrophotography it's important to remember the lens aperture (not f-stop) will define its limiting stellar magnitude.
Being mindful of this, I have a fisheye (see attached) headed my way from Japan and am quite keen on seeing what sort of a job it will do on the southern Milky Way as its front element is nearly 3x the diameter of my current lens.