I would totally go for the Tokina.
The sigma is a good lens, but not a great lens. It has pretty good contrast and color rendition, but it's less sharp, it has some super ugly bokeh, and the distortion is very complex (impossible to correct) and can be super ugly. I've seen images where a ski is in the corner and it appears to be a foot longer than it is. It seems to me like Sigma designed their lens to pull some extra wide towards the edges by severely distorting them. It's not very apparent in landscapes, but can be super apparent in other places, especially skiing.
Here's what I'm talking about:
I know some people hate Ken's reviews, but when you are comparing objective information (in this case, sample shots), it's very useful.
Besides that, they feel a lot cheaper than the Tokina.
I needed a super wide quick, and I bought the Tokina despite my general dislike of 3rd party glass. Tokina has recently been making me re-evaluate that opinion.