Tighearnach - Well, I love my Tamron 17-20 and Sigma 10-20 ... don't have experience with the canon 70-200, tho.
The only disadvantage I've found of the Tamron is that it is not a USM lens ... a bit slower and louder than the ultrasonic lenses, but not much and it hasn't hurt me a bit. Build and IQ are excellent.
I compared the Sigma to the canon 10-22 -- liked the Sigma better and have been very happy with it. http://www.10-20mm.com/