The Tokina 17-35/4.0 i supposed to be really solid with nearly ZERO distortion!! to me, it looks close to both of your mentioned lens.
I never had the 16-35, have a 17-40 that's for sale.
I found it to be pretty usable at the wide end if stopped down to f/8 or smaller, improving considerably as you move to the long end.
If I used it for landscape work and didn't focus at hyperfocal or closer, my lens was always soft in the corners. Worked well for indoor and other close-focus material, slightly disappointing if I wanted crisp-to-the-corners large landscape prints.
I got the Tokina 17-35/4 and it's excellent in many ways from 21-35mm but the corners at the wide end are as bad or worse than the 17-40, depending on how you're using it so not much of an improvement, if any, on Canon, but an option in F-mount.
I'm currently trying to put together a wide-zoom kit for Nikon landscape work, minimizing overlap and maximizing performance. The Tokina 16-35 and 16-28 are both in the running for the mid-range wide-angle zoom with the Nikon 14-24 covering its best from about 14 to 20mm.