I wrote Bryan and asked him his preference between the two. He responded and said that he has gotten the question enough that he will do a writeup shortly on the subject.
IQ is huge to me, but there is more to a lens than that. Sometimes it is about the quality of the image that can't be determined on a chart. I'm very interested in some direction comparisons of subjects other than charts or walls.
Ditto, very keen to see the difference in the real world the hybrid IS and small size are super tempting
Me, too. I have to confess that while I love the look of the Sigma and its finish, I don't need another prime in my bag that large. It is not smaller than my standard zoom (Tamron 24-70). I used the original 35mm f/2 as a portable lens that could still provide narrow depth of field and close focus work (I have been using the new shorty forty more for that now, although f/2.8 is not f/2.)
The Sigma would be great if you were using it in, say, a holy trinity kit. But I've already got 8 lenses in my modern kit, plus another 6 or 7 manual focus primes that are in my rotation. Smaller size is good in my book. That being said, I can see the substantial build of the Sigma being a bigger plus for someone else.
Bottom line: IQ is what matters most to me. If the Sigma is significantly better (not just sharper), it gets my vote and perhaps my cash. If the rendering on the Canon is better, I may lean that way.