The quality of the Sigma 10-20 seems generally very good, although Sigma do suffer a little from QC problems, so it depends on whether you get a good one. The Canon 10-22 is more expensive but will probably hold its resale value a little better. I have the Canon and am pleased with the results, although I'm not sure if I'd have bought it in preference to the Sigma (or Tamron, or Tokina) if I hadn't had a cash-back voucher which knocked the price down to about the same as the third-party lenses.
As far as I can see from reviews, the only significant advantage that the Canon 10-22 may have is lower distortion, especially at the extremes of focal length. But as always, you pays your money and takes your choice.
