I don't have any experience with either, but when I was looking last year I did quite a lot of research and the Sigma was very highly regarded. I was tossing up between it and the Canon 50mm 1.4. After much continual changing of my mind, in the end I went for the canon as the smaller size was the tipping point.

I am guessing you have seen this

Sigma 50mm F1.4 EX DG HSM review: Digital Photography Review