I am considering the Sigma 12-24 HSM II. I need something wider than my 17-40. The EF 14mm f/2.8L is out for two reasons: fixed FL & price. I looked at the Rokinon/Samyang/Bower 14mm f/2.8, but would rather have a zoom in this range, if possible; 12mm is considerably wider than 14mm. The price is right, though.
A 10 or 12mm to 20 or 24 mm, manual focus lens would be perfect. I don't get why you'd even put AF in this (the Sigma) lens. I'll be shooting it for maximum DOF, most likely at f/8 and smaller, so I don't much care what the maximum aperture is. It will be used on a 6D.