Pro/Cons for Sigma 12-24 II & Canon 16-35 II

I'm looking for a wide angle zoom and I narrowed my search to these two lenses. I intend to use them on a full frame (5d3) mostly for landscapes but some architecture as well.

The main criterion of selection is sharpness. A prime would do a better job but, as an amateur I can’t justify the price of a good prime, plus the zoom is handy.

Distortions are also important, but I don’t mind PP. As long as distortions can be corrected I can live with them. 12mm sounds very tempting but once corrected for distortions how close does it get to 16mm?

The aperture doesn't really matter as they will be mainly used at F8 or slower. Also, I don’t intend to use either lens at 24mm or more as I have better glass for that.

Being able to use filters and weather protection are a plus to Canon.

Photozone doesn't praise much 16-35 II in their review (actually about as much as 12-24 II). The-digital-picture doesn't have Sigma mk2 in their IQ charts to compare it with Canon.

What are your opinions? I hardly expect anyone to own both but do you think Sigma is sharp enough? Do you have to crop to compensate for distortions? Is Canon just ok or is it top class L glass as the name (and the price) suggests? Is it a no brainer to buy this third-party lens when Canon has the best? Or am I throwing money at Canon when there’s a much wider zoom out there with similar IQ?

Cheers!