I did a head-to-head comparison between the Canon 35 f/1.4L and the Sigma 35 f/1.4 DG HSM. I looked at the strengths and weaknesses and wrote up some thoughts. on when one lens might be a better choice over the other
Basically, the 35L has a more robust construction, while the Sigma is sharper. Focusing is about the same in both lenses. Bokeh on the Canon better, but in the end the Sigma's image quality made it the clear choice. You can take a look at the the rest of the writeup if you're curious about the details and let me know what you think.