I have the Sigma 10-20 (the older one!) and I really like it - seriously wide. Very sharp and I like the build quality of it.
I can't say anything about the Tamron but I can say that I'm more than happy with my Sigma!
Have you considered the Tokina 12-24 f4 - I had one on a d90 & D300s much prefered it over the Siggie......
In the latest N-Photo Nikon magazine they did a group test on ultra wide lenses. Out of ten lenses only the Nikon 12-24 and the Sigma 10-20 4-5.6 attained five stars for image quality. The Sigma is a couple of hundred quid cheaper. All the tested lenses were for crop sensors.
Curious to know what the Tokina rating was then..... If you know :shrug: