i have the canon and its very good, after reading reviews i kind of ruled the sigma out, the canon is more expensive but seems to be a lot more consistent, the sigma is one of those where its pot luck as to whether you get a decent one.
I also find that the canon gives very little distortion, something that people posting pics of the sigma seems to suffer from.
But then when i suggested on here that my canon didn't seem to suffer from distortion I was gunned down, that was from the nikon crowd though.
I would lend you mine but i think i'm a bit far away and i'm not keen on sending it through the post being so expensive and delicate.