I tested Canon 35 f1.4 (that I own) and Sigma 35 f1.4 (that I jumped to buy immediately) and Canon lens has a more pleasing bokeh and image. I made the comparison on screen and paper (1000mmx700mm printed images).
Truly, the Sigma looks crisper, but I prefer a general beter looking image.
I made this comparison because I could sell my used Canon 35 f1.4 at the price of a new Sigma 35 f1.4 and I almost made the switch, but then I said to try the Sigma first.
I decided to keep my Canon lens from two reasons:
1. Sigma did not demonstrated yet that can make a working horse (I had a Sigma 50mm f1.4 and was extremely unreliable). I can`t afford to have bad working lenses or on repair for weeks in a row. This will be a disaster for me.
2. The images printed looked beter from Canon. The colours and general image are more appealing.
In the last years we are all looking at the numbers, but nobody looks at pictures anymore.
I got the idea that the Sigma is cheap, but I do not want cheap, I want the best Image and reliability.
And I am sure that all of you know what I am talking about.