Based on the majority of reviews I have read that compare the two lenses, that's pretty much the only good reason to pick the Canon.
And if you pick the Canon, the Sigma 10-20 is very close behind for much less $$$. The Tokina 12-24 is very nice and sharp, sharper than the 10-20, and competes well with the 10-22. Sigma's 12-24 is nice too, but for the price, unless you are planning to go FF I think the Tokina 12-24 is nicer. The Tamron 10-24 is the softest of all of them... love the versatility of the zoom range though.
If you are ok with losing 1mm on the wide end, I would strongly consider the Tokina 12-24 if the 11-16 is too short for your style of photography.
thanks for your input. I will have to look at the siggy. I really need an uwa for interior shots so the 12-24 wouldn't work as well i don't think. also I have the 15-85 which i'm pretty happy with.
There is another good reason. I find the flare on the Tokina a lot worse than the Canon. That, and the limited range, is the reason I'll be keeping the Canon for my everyday UWA.
I did notice that when reading all the reviews.