I have neither, but several pros I know do have the Sigma and they say it's excellent.
The Sigma is the equivalent of the 80-200 f/2.8 AF
-S Nikkor (it also has lensmounted motors), while costing less than the non AF
-S Nikkor yet provides almost the same optical quality.
Even if it is 1% less, at 50% of the price it's a far better deal...
Sam, you can't blame Tokina for not building a lens that is indestructible.
I have several Tokinas (including an ATX-Pro 28-70) and they're very well built and focus quickly and accurately.
Of course an AF
-S Nikkor is faster to focus, but an AF
-S Nikkor is also faster than an AF
-D Nikkor. For the money Tokina AT
-X can't be beat, and I dare to state that they're better than anything except AF
-ED Nikkors yet cost about as much as non AF
Given the choice between a standard (non AF
-S) Nikkor, a Tokina AT
-X and a Sigma EX I'd go for the Tokina first, Nikkor 2nd and Sigma a very close 3rd (with the 70-200 Sigma coming before the AF
-D 80-200 Nikkor because of its HSM motors).