Hi,
Was in the same situation as you a while ago, looking for a wide angle lens to complement my 18-200 VR. Tested both of them in OP and found image quality was wonderful for both, nearly impossible to tell apart, but the Sigma was probably just a little sharper to my eyes after some serious pixel peeping. In terms of built quality, Tokina feels more solid probably because it's heavier, but the Sigma's not too bad either, considering that it weighs significantly lighter.
In the end, went with the Sigma just for its 2mm wider range, which is quite significant IMO, especially when the whole point of getting an ultra wide is just that, to get as wide as possible. You probably won't go wrong with either as they're both excellent lenses, I'd suggest you go down to the shop and try both out to see which you prefer.
Regards