Hi Ian, this one comes up a lot, I did a lot of research before making my choice.
Everybody seems to recommend the one they got, so basically they are all good, and its hard to make a bad choice.
However, a significant minority of people got poor copies of a Sigma 10-20. Usually soft to one side, especially at max aperture. Many have got good copies of it, on which case its a bargain. I didn't consider it as I could afford not to take the risk. Only buy from somewhere you can return it to. Note that there are TWO Sigma 10-20 versions. Probably avoid the older variable aperture one, the other is a constant zoom f3.5
So pick your most important feature.
Range - Canon (or Tamron)
Speed - Tokina 11-16
Wide - 8-16mm Sigma
Price - 10-20 Sigma
The general consensus is that the Tokina 11-16 is really good, sharp and the fastest in its class. This is probably the best choice for most.
Personally I went for the Sigma 8-16mm which is also sharp, wider, and well reviewed, although a lot slower, and you can't use filters.
This is the most comprehensive and in depth group review I've seen are here:
http://www.the-digital-picture.com/Reviews/Sigma-8-16mm-f-4.5-5.6-DC-HSM-Le...
http://www.the-digital-picture.com/Reviews/Tokina-11-16mm-f-2.8-AT-X-Pro-DX...
http://www.techradar.com/news/photography-video-capture/cameras/best-wide-a...
http://www.techradar.com/news/photography-video-capture/cameras/best-wide-a...
see this thread too
http://www.ukclimbing.com/forums/t.php?t=528156
You'll love the 600D, they are a real step up from the 450D generation.