I was in the same spot a week ago. Tokina is sharper, and it's a great piece of glass. You can check out flickr galleries to get an idea of what to expect in terms of flares. If ND, GND filter is your thing, then your best bet are the filter holder systems from Lucroit, and Fotodiox for the Tokina 16-28. Also, I cant remember clearly, but the sunstars from Tokina looked nicer to me at least.
I ended up getting a used Canon 16-35 II today, so will give it a try and if I dont like it then I will sell it off for something else. Another lens that you can look into is the Samyang 14mm. It's sharp, and a great piece of glass and costs only $350. It's only downside is the complex mustache distortion but can be fixed in pp. If you are doing long exposures at night to capture milky ways, and stars then you might want to look into which of the lenses handle coma better.