Cheers Mike. I agree that the extra sharpness of the Canon is probably not noticeable most of the time. I would feel more comfortable with the Canon build quality, but the main reason I wouldn't swap my 10-22 for a Sigma 10-20 is the extra 2mm. It makes a huge difference if, as in my case, your next lens up is a 24-105 (or similar). Even when I had an 18-55, I used that extra length quite a lot - anything that saves lens swapping is a good idea!
So the Tamron 10-24 would be even better ...... except that it gets very poor reviews. I'd love the extra length, but not at the cost of poor IQ.
But at those prices you quote ..... hmmmm ... it's a big difference.
PS: no point whatever in having constant f/3.5. With any ultra-wide, you could glue the aperture down to f/8 and practically never change it unless the light is really bad and you are hand-holding, in which case you just go to whatever wide-open happens to be.
Enjoy your new lens!