I have the nikon version of the sigma 8-16, while my brother has the tokina 11-16. I love both lenses. My brother's tokina is slightly sharper than the sigma, and at F2.8 great for astro-photography. However, the range is limited and shows significant lens flare. 8mm on the sigma is great (big difference in my opinion compared to 11mm). Surprisingly, it suffers from lens flare significantly less than the tokina. Its main weakness (compared to the tokina) is that it is a slow lens. Also my copy focuses inconsistently on my cameras.
The new version of the Tokina has new coatings and may have tackled the lens flare issues, but I have not seen the reviews as yet. Quite frankly I don't think you will go wrong with either but it depends on your use. If you really want 8mm the sigma is the one to get, but if you want f2.8 get the tokina.