Not being a Canon person .. I the first thought that popped into my head was the question .. do you use filters of any type? ... Or do you prefer the digital filter and/or processing methodology?

Andrew(I@M) has the Tokina 16-28, and from what I've seen it's mighty impressive.
I don't think that it's ability and quality should be in question.

It's just that it has a massive front element, and no filter threads .. hence can't take filters.*

* possibly could if you found some addon contraption, that may or may not be available for purchase .. or manufactured something to suit.


Also, are you referring to the version 1 of the C 16-35/2.8, or the II series lens?
I think they are different, series II appears to be better in terms of overall IQ.

Looking at TDP and PZ's tests .. technically the Tokina is slightly better, in terms of overall image quality .. but the two are so close in how they create an image, it'd be impossible to determine which lens took which image.

Have to say too tho, the Canon 16-35/4 IS seems to be the overall winner here tho!(apologies for the spanner in the works too)
If it were for landscapes .. I'd say probably the best choice. If it's also for low light wide field imaging(eg. starry nights .. or party events) then of course the slower aperture is a handicap.