I think you mean the Sigma 10-20 and the Canon 10-22. You can find some comparisons by fStopJoJo and other review sites. Basically the Sigma is close in quality to the Canon (some say better) but cheaper.
I went with the Sigma because I regard this range as a bit specialized/novelty, and the Sigma is about 1/2 price here in Australia. The Sigma has been great. High quality, solid, I think it's great value.
I believe Sigma had some quality control issues early on but most reviews I've read have a lot of good things to say about the Sigma. I don't regret not going with the Canon at all, and it's my only non-Canon lens (except for the Tamron I'm selling)
Malcolm