I have the Tokina, but was close to getting the Sigma 8-16mm.
The main reason I didn't go for the Sigma, was because being an ultra wide angle lens, I wanted to use it for landscapes, and as you can't put a filter (CP, ND etc.) on the Sigma, it would be a bit useless for me.
I also like to take indoor shots without flash, and the 2.8 of the Tokina sealed the deal.
The IQ of the Tokina is excellent, as is its depth of field, and the colours and contrast are also excellent and as good as any of my L lenses.
The new model just adds faster focussing, and to be frank with you, because of the deep DOF, you hardly need to focus anyway and a lot of my work is done in manual focussing anyway.