i'm trying to decide in this segment too. in short:
canon: best all-rounder
sigma 10-20 f4-5.6: also good al-rounder
sigma 10-20 f/3.5: better centres, faster aperture, mushy corners at 10mm
tokina 11-16 f/2.8: fast aperture
sigma 8-6: widest and slowest.

For me, it's down to the tokina and sigma 8-16. both seem to have (from reviews i've read) better IQ and less CA than the canon. focussing won't mean much to me because i MF a lot, and even with an f/2.8 aperture and hyperfocal of a few meters i'd probably set up on a tripod for night shots at f/5.6 or so, so the tokina loses its advatages and i'm leaning towards the sigma 8-16. but it's such a tight race between those two and the canon that i'm just going to buy whichever comes along at the best price at the time...
(or wait for the 'does-it-all' 8-20mm f/2.8 with better IQ than all of them and costs twice as much as the others)