I am deciding to stay with Canon, after much thought (not too much actually). I use the 17-40L on my 5d2, which is pretty decent. I want to upgrade to a sharper lens, but don't look forward to losing: 1) Autofocus 2) zoom range or 3) ability to use filters. Maybe that's why, even at $699 on sale, I have not bought the Tokina 16-28.
The Zeiss glass (especially 21/2.8 Distagon) is supposedly a phenomenal lens. The 18mm is rated very highly, too. I know, I know, I can rent these and try them out. If I rented a wide angle, it would only be one i didn't already have or really plan to buy. Maybe I can rent the Zeiss 18mm since finding a 21mm used for under $1,300 seems impossible.
The Tokina is most in line to what I need, at least on paper. But with limited sample photos and reviews, the decision becomes much less clear. I believe it is probably very similar in IQ to the 17-40 I already own. I know the distortion control is very good on the tokina, but i need sharpness across the frame!! Please share your experiences, if any. Thanks