As a pro shooter get the 16-35 f2.8 ii. The extra stop of light and general improvement before f11 over the 17-40 f4 is well worth it, especially at corporate events and weddings where you have to work in available light which is often far from ideal.
If something far better which will suit your style of shooting comes along you can always sell, and L lenses tend to hold their value pretty well (far better than 3rd party lenses, although this does mean you're more likely to pick up a bargain 2nd hand tokina)... But the point is, as a professional you should invest in the equipment you need to do the job today, not wait for something which may do the job better in a year or two, the difference in cost between the 17-40 and 16-35 is less than one wedding/event.
As has been mentioned here the potential issue with both the tokina 16-28 and the posited 14-24 is that they feature bulbous front elements which mean that you wont be using screw on filters - so its a mattebox or nothing. No problem for landscapes, but not what you to have to be fiddling about with when running around at a wedding or corporate event... And you describe yourself as a pro who does weddings and events... The samyang/rokinon is not what you want for the type of work you describe (where versatility is key).