In all honestly, the Tokina is only a few points off from the Canon and when you consider everything that modern software like Adobe Lightroom can do, it will be very easy for no one to be able to tell the difference between the lenses because professional photographers are going to apply the lens correction.
But then you consider the price. Tokina’s option is under $1,000 and Canon’s is pretty much twice that price. Canon’s lens offering though is better built and has weather sealing. Tokina’s doesn’t. So if you don’t need weather sealing because someone at the wedding you’re shooting has been drinking too much and spilled on you, then the Tokina makes a lot of sense.