I've used both the Sigma and Tokina 11-16 - and ended up getting the Tokina. Never regretted it for a second.
Solid build, confidence-inspiring action and optical quality very very satisfying.
It's a little more expensive than the Sigma, but n money was spent on an AF motor, hence the higher quality of everything else in the lens.
The constant F2.8 aperture is superb for those indoor dim-light shots where you can only go hand held.
A curious coincidence: yesterday I had the opportunity to do some comparison shots between the two, swapping lenses and taking the same shots. The Tokina is visibly sharper in the corners and I also noticed that the colors appear better out of the Tokina. Not a scientific study by any means, just observations. It wasn't intended as a serious test, since I bought the Tokina some time ago - but fun to see what the Sigma could do. The Sigma is not bad at all and is 1mm wider.