Joe,
I currently have both of these lenses and I have attached a few comparison shots I took the other day, a very quick and dirty test, done in the kitchen, I placed the subjects straight in front of the window to deliberately get a high contrast effect and to see how both lenses fared with chromatic aberration which I have found prevalent with the Sigma.
Putting it simply, the Canon does to me have the edge and although some reviews say its soft at F/1.4, I have not as yet noticed it or been bothered by any issues, I will however probably err on the side of the Sigma, purely because its smaller and lighter and thus easier on my injured arm.
The Canon appears brighter more evenly across the frame, and suffers less overall with chromatic aberration, build quality on the Canon is as they say, addictively excellent, the Sigma does pick up more chromatic aberration in moderate use which is irritating but correctable, but both are great sharp lenses.
You can see the original cropped photos for a better and easier comparison by
clicking here:
Photo 1 Canon 35mm F/1.4 L v Sigma 30 F1.4 at F1.4
Photo 1 Canon 35mm F/1.4 L v Sigma 30 F1.4 at F2.8
Photo 1 Canon 35mm F/1.4 L v Sigma 30 F1.4 at F4
Photo 2 these are a 100% crop of the centre section of the ingredients of a bottle and the Canon is more than acceptably sharp..... You can see the original complete shot by following the link above.
Photo 2 Canon 35mm F/1.4 L v Sigma 30 F1.4 at F/1.4
Photo 2 Canon 35mm F/1.4 L v Sigma 30 F1.4 at F/2.8
Photo 2 Canon 35mm F/1.4 L v Sigma 30 F1.4 at F/4
Hope this helps, if you get one, you will not be disappointed.
Regards
Roy