Upload & Sell: Off
I've got the Tokina 12-24/4 and I love it. I previously shot with a Canon 10-22 and I think the build quality is better on the Tokina and image quality is a toss up. 10 is definitely wider than 12, but I'm not sure if it's worth the price premium. I've been very happy with my 12-24.
For those who are worried about f4 vs f2.8 -- what kind of shooting do you do? You might need 2.8 if you mostly shoot concert stuff, but remember that a fast aperture is less important on a wide lens. You won't be getting a thin DoF anyway, and at 12mm you only need 1/24th shutter speed (using the 1/2*focal length rule).