I had the Canon and I experimented with the Tokina before buying it.
I think the Canon is a better choice due to excellent IQ, wider FL, surprisingly little distortion for an ultrawide, full time manual override, and now you can get it for great prices (although that totally throws the resale value argument out the window, it has probably gone as low as it can go).
Unless you think f/2.8 is worth trading ALL the above for, the answer is clear to me (ok, except the IQ- they are more or less at par).