I own both the Canon and Tokina UWAs. Which is best depends on which conditions you'll be shooting in. For anything outdoors in the sunshine then the Canon's supreme flare-resistance makes it the lens of choice. For interior shooting or for astronomy wide-fields then the f2.8 of the Tokina is the deciding factor.
These are really the key factors you have to think of before picking the right lens.
I have extensive experience with both lenses and Frank sums it up really well.
Tokina is fast, sharp, produces awesome spikes when closed down but performs really bad when shooting straight into the sun (dont have experience with it shooting into strong spotlights on concerts).
Canon is sharp, more flare resistant, have a broader range but is slow which can be a dealbreaker where 1-2 stops can make or brake a shot.