I recently purchased the Canon 15mm fisheye. Great lens. Very sharp, even after distortion correction. Much better than my previous Canon 14mm/2.8 L lens, especially on a 1Ds. I personally haven't seen a side by side comparison between the Canon and Sigma fisheyes, so I can't comment on equivalencies. The build of the Canon is solid, so I can't see how the Sigma would be that much better (haven't checked one out, however). The only reason I went with the Canon over the Sigma was due to my personal experience w/ other Sigma lenses having slower and/or less precise focusing than similar/same Canon lenses. This is definitely my own personal, anecdotal bias...and I'm not presenting it as fact. Either lens will likely give you everything you want. Just be certain you purchase from a source that allows reasonable returns, so if you do get a dud, you can either exchange for a different copy of the same lens, or the alternative lens if you're so inclined.