I've now used both and I have to say the Canon gives better IQ overall, but thats simply because it uses better glass. Some Sigmas are better than other Sigmas which doesnt help!
The general consensus (web and magazine wise) is that the Canon is the better lens, but you always have to way it up against the price. Is the extra worth it? In my opinion it was, as I wanted the best UWA I could get, and if I bought the Sigma I know I'd still be hankering after the Canon.
I'm currently battling this debate again in relation to buying a Canon 70-200L, against a Sigma 300EX!