I have rented both lenses. I have already posted samples and impressions on the canon 35 and I am using the sigma now. Both are unique lenses. The sigma really is sharp wide open and I don't believe the 35L is. i'll post some samples with the sigma later . Bottom line is there is no wrong choice. It really comes down to IS vs. the extra stop, and it is a usable stop. The sigma is a bit sharper at f2 but the canon is very usable wide open.
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