I owned a Tokina 12-24 and sold it in favor of the Sigma 10-20 and never looked back.
The extra 2mm dont sound like much, but it makes a world of a difference in UWA terms.
I ran quite a few tests and sharpness-wise they are very similar.
Just wanted those extra 2mm, and am happy with it.
Whichever one you choose, you are not going to be unhappy with either, so go for it
