I had the older version of the sigma 10-20 (f4-5.6) and I replaced it with the tokina 12-24.
For me, I was happier with the output from the tokina because it was generally sharper across the frame compared to my copy of the sigma (but with sigma each copy can be really different).
Here's some samples, but not really a fair comparison since these were shot on different dates so light was different and the settings weren't the same.
Sigma 10-20,
10 mm, 1/60, f5.6, iso 800
Tokina 12-24,
12mm, 1/80, f5.6,iso 400
Sigma 10-20
10mm, 1/500, f11, iso 100
Tokina 12-24
12mm, 1/320, f11, iso 100
Sigma 10-20
12mm, 1/30, f25, iso 200
Tokina 12-24
12mm, 1/160, f9, iso 400
Sigma 10-20
Tokina 12-24