I have experience with the Canon 28 f1.8, Canon 50 f1.4 and the Sigma 50 f1.4 but my experience is on a full frame camera.
I think the 28mm f1.8 is pretty good, there is a bit of CA and distortion near the edges of frame when shooting stars wide open but that's probably not a big deal on a crop camera.
examples with this lens on 5DmkII here:
https://hughsblog.wordpress.com/2014...ora-australis/
Of the 50s the Sigma is much better in my experience. The Canon example I've used was soft with bad CA whereas the Sigma one is nice and sharp with very little CA.
I know it's a bit slower but I really like the Canon 40mm f2.8 STM. It's an awesome walk-around lens and, though it vignettes a bit on a full-frame, stars are sharp all the way to the edge of frame at f2.8. Down sides for astro work are, of course, smaller aperature of f2.8 and because the focus ring drives a stepper motor and isn't mechanically linked to the focus you can't mark an infinity mark, though this isn't a big deal if you're going to focus with liveview anyway.