I tried the 11-16... that 5mm range is so little that you may as well leave it on 11 and use it as a wide angle prime (though its not as good as a prime).
The ONLY thing the Tokina has going for it is the F/2.8 aperture. The Sigma 10-20mm is a way more useable lens.
Look at it this way... 90% of the time, we want wide angles because we are outside. Outside, we don't really want shallow DOF, we want deeper ones, so we are going to up our apertures to F/8 or higher (numerically speaking), and at these levels, the Sigma is just as good, if not better... and far cheaper, than the Tokina. It also has double the range, and a 1mm more advantage over the Tokina (in real life, read that as no advantage, but it looks good on paper).
The Sigma gives fantastic results with rich tones:

(Click on the photo above for a larger view of the same shot)
To add, Sigma has a newer version of this lens that is a constant F/3.5, which is a mere 1/3rd of a stop less than F/2.8... plus it has the other advantages over the Tokina.
Me, I have the 10-20mm variable aperture, and the picture above was taken with this picture... it is a VERY good lens.

(Click on the photo above for a larger view of the same shot)