You should take a look at this lens instead of either of those:
http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=351542&is=USA&addedTroughType=categoryNavigation
Canon 10mm-22mm f3.5
It's not one of their pro lenses, but it's designed for the digital SLR cameras so that it's roughly equivalent to the fisheye. If you put the 15mm fisheye on a digital SLR, it acts like a 20mm on a 35mm film SLR.
Canon doesn't share it's tech info with other lens makers, so Sigma has to go out and buy a bunch of Canon cameras and lenses and try to reverse engineer them by taking them apart. Then Sigma (or any other 3rd party) will try to use as many common parts that they use on other lenses to save money. End result is that they come up with a cheaper version that isn't as good as the Canon lens which is engineered to only work on Canon cameras.
Buy the Canon lens.