Canon is a bit better, but is it £200 better?
I bought my sigma a couple of years ago from the amazing B&H in NewYork. They have 50 assistants behind the DSLR counter alone!
I went in to buy the Canon but the bloke behind the counter talked me into buying the Sigma despite the fact that it was cheaper. He reckoned better optics, better coatings, and hood included. I have heard the build quality critisism before, but it seems Ok to me and I have got an L lens to compare it too.
I bought the sigma, didn't think it was sharp so took it back and got the canon instead - that lens is sharp as hell. Don't regret paying the extra. More regret fiddlng around in the first place thinking I could go for the cheaper option.
I've tried both, I found the Canon sharper across the whole range than the Sigma, but the Canon is very, very sharp.
I bought the Canon as it definately had the edge on IQ especially looking at 100% crops, and I simply wanted the best UWA I could get, which for a cropped sensor Canon is the 10-22. Was it worth the extra?? All I know is that having used both, if I bought the Siggy (which was still very very good) I'd still want the Canon
That said, the Sigma is exceptional value for money!
Looked at the Tokina 12-24? Optics of the Canon (almost), faster, better built and still the same price as the Sigma. With the added benefit of not having to worry about a dud when ordering online.
I tried all three in a shop before getting mine and IMO the Tokina is the best built (by far), and the Canon has the better IQ (only just from the Tokina, and reasonable amount more than the Sigma). I went for the Tokina as there wasn't much in it from the Canon, but it was around £200 cheaper.