I know that the Tokina lenses I've looked at for my Nikon range from crap on their low end, to some really nice lenses on the high end. All three of those Nikon lenses you listed are very nice lenses. The only reason I would go with the Tokina wide angle over the Nikon wide angle is the lower aperature.
A website that has always been my favorite to see comparisons of lenses is
http://www.dpreview.com/ and specifically their
Nikon SLR lens discussion page.
Just doing a quick search for the Tokina 11-16 yielded these two interesting threads with some picture examples:
Thread 1 w/ pictures
Thread 2
Edit: As noted in one of those threads, with the Tokina lens you're basically getting a lens that will perform at about 95% of the picture quality as the Nikon, but for $300 cheaper. Worth it in my opinion.