Haven't used the Tokina, but photozone.de has reviewed it. Better than the Canon 16-35 II for distortion and vignetting, slightly better center resolution but the Canon is much better in the corners until both are stopped down to f/5.6, and the Canon is much better on CA.
Canon is also weather sealed and takes filters. I use a B+W XS-Pro UV to complete the weather sealing, a B+W Käsemann CPL sometimes (at wider focal lengths, the sky is unevenly polarized), and a Schneider Optics (B+W's parent company) 10-stop ND for long exposures, especially helpful to eliminate people from shots. 82mm filters are expensive, but at least I can share the CPL and ND with my TS-E 24mm II.
I definitely agree with the utility of f/2.8 for evening/night handheld shooting, especially with the high ISO capability of FF. Here's an example:
EOS 5D Mark II, EF 16-35mm f/2.8L
II USM @ 27mm, 1/25 s, f/2.8, ISO 3200