I used to have Tokina 12-24 and was happy with it. CA was a pronounced issue with the lens but unless it was really contrasty situations, it did not bother me.
With regards to canon vs sigma, I would go with canon unless an FF camera is on the horizon. If I am not mistaken, sigma is compatible with FF whereas canon is an aps-c lens.
Kernuak - I'm visiting Tromso and will post some polar summer images when back..