Are the Sigmas better than the Canons? Where can I get em cheap?
Massively, epically no.
However, if you go with Canon they don't have aperture rings so you'll have to buy an adapter that has one.
Older manual focus Nikons have aperture rings, allowing you to buy a much less expensive adapter.
Nikon's 28mm 2.8 AI-s is excellent (supposedly the sharpest wide angle they've ever made), as is the 50 1.2 AI-s. I haven't used the 24.