As I said, the IQ from the Canon is very high. I never used the Sigma.
If you want, send me a PM and I'll Dropbox you some Canon 15mm RAW files.
I pulled the trigger and purchsed a Canon EF 15mm 2.8 used but in excellent condition. As you said, contrast and sharpness are really very good and better than the sigma counterpart. The only BUT is CA that is very pronounced (magenta color fringe) at the borders of the image so, I have to correct them in LR to make them perfect.
So far, I am not really satisfied but I guess it is the best option at this cost.