I'd happily say "go Tokina".  I didn't buy one myself (when there was a lovely
used one up for sale a few years ago) I went for a new OEM instead.  On the web
I had found a few mentions that the Tokina had some issues with lubricant
migration.  I don't know that this is true or not, but it did give me pause.

That particular Sigma should be a bit better for sharpness/a bit worse for
distortion than the Tokina, but Sigma build quality is questionable (the ones
I've seen and held which does not include the one above).

Tokina: distortion: -1.28 to 2.69%, 3.4/5.0 for wtd sharpness
 Sigma:             -2.22 to 3.79%, 3.9/5.0 for wtd sharpenss

(source: photodo).

G'luck,
Alan.