1) Build Quality--Even
Both lenses are well built and solid feeling. For my purposes, there were no differences noticeable enough to me to make any difference. The only thing that was a bit of a niggle for me was the lens cap for the Sigma--what a P.O.S.!!!

2) Focus--Sigma
Both lenses were very accurate with their focusing, but the definite edge for me goes to the Sigma because oft he AFS style focusing. It is for sure faster, and you can manually override the focus on the Sigma once AF has been achieved by simply turning the focus ring on the lens. On the Tokina, you must either switch the MF/AF switch ono the body, or "pop" the focus ring on the lens.

3) Flare--Even
I did not notice any situations where either of these lenses exhibited any unnecessary or surprising flare. Both come with lens hoods, and it is worth noting that the Sigma's hood is MUCH smaller than the Tokina's. Both seemed to work effectively enough though.

4) CA--Sigma
The Tokina definitely exhibits some consistent CA. In the same shots, it was simply not present with the Sigma. Whether it is enough to show up on prints, I do not know, but it is there.

100% Crop Comparison:
http://gov.smugmug.com/photos/43850992-L.jpg

5) Vignetting—Tokina
Distortion—Sigma (Slightly)
I never have shot a brick wall before, and I don’t plan on doing it again. I chose to do so, because it was effective for showing the vignetting that might be hard to ascertain from an everyday shot. For distortion however, it is tough to get everything perfectly square, so draw what you will from these shots. I can say with pretty strong confidence that both are surprisingly good at handling distortion, with a very slight edge towards the Sigma. Both lenses exhibit some vignetting, but the Sigma's is definitely more than the Tokina.

12mm wide open:
http://gov.smugmug.com/photos/43853038-L.jpg
12mm f/8:
http://gov.smugmug.com/photos/43853042-L.jpg

20mm wide open:
http://gov.smugmug.com/photos/43853045-L.jpg
20mm f/8:
http://gov.smugmug.com/photos/43853046-L.jpg