A comparative report in the Canadian magazine Photo Life revealed that
by comparison...
1. The Sigma was a bit sharper overall than the Tokina at all apertures.
2. The Tokina had better flare control than the Sigma thanks to a longer
lens hood.
3. The Tokina was a bit more sturdily constructed than the Sigma, which
is still well built.
4. The Tokina is significantly smaller and lighter than the Sigma; hence
it is easier to carry around in the field.
Other reviews seem to indicate that the Sigma has better close-up
performance, although the Tokina's is pretty good. And it appears that
the manual focusing ring in the Sigma is better damped than the
Tokina's. One person feels that the Tokina produces nicer colours in
photographs than the Sigma.
I'm only relaying what I've read about. I haven't used either of these
lenses, so I'm going strictly on hearsay. I think that either of those
lenses would be an improvement over the super-tele zooms that are out
there. I've used the Tamron 200-400 and wasn't satisfied with its
performance at 400mm.
Craig Yuill