These sides-by-sides were shot less than one minute apart on a mostly sunny day.  The lighting is slightly different, but not so much that the comparison isn't valid.  Both lenses were set at f/11 for my first test.  I wanted to be certain that most of the foreground was in focus for sharpness comparisons. The Tokina was set at its 24mm point, which was pretty close to the Nikon's 24 as you can see in the full frame shots below.

I plan to do some wider f-stop comparisons when I get the opportunity.   My basic conclusion (albeit from only one photo!!) is that both lenses seem to have comparable contrast and color qualities -- this was more important to me than edge sharpness.  In fact, I couldn't see any real contrast/color differences in blind large views in PhotoShop.  The Nikon Prime is a touch sharper at the edges, and the Tokina Zoom exhibited much more blue/yellow ghosting (chroma?) in high-contrast areas at the edges of the frame.  This is particularly apparent in the sky zoom.  Keep in mind we are comparing a Zoom to a Prime!   I'd love to know whether the Nikon 20-35 Zoom exhibits any of this ghosting at the edges. I can't see any contrast, color, or sharpness differences in the center portions of the frame; if anything, the Tokina shot looks a little sharper!    I have to say that, so far, it seems the Tokina is a good lens both mechanically and optically, and a great value at $630 vs $1500-1600 for the Nikon zoom.   It certainly held its own well against the Nikon Prime. 

Here are the shooting and scanning details, along with the full-frame shots and zooms...

Exposure:  f11 and AE with +1EV for snow.
Film:  Velvia exposed at iso 50 in a Nikon N70.
Filters: none
Tripod mounted.
Scanning:  Nikon LS-2000 at 2700 dpi
Processing:  PhotoShop 5.0; equal level adjustments; equal unsharp masking.