With all 3rd party lenses, I'd stick with their top lines only (or with lenses that you can't get elsewhere).
The low-end lenses cannot yet compete in quality with the equivalents from Nikon or Canon, the top lenses can (or at least get very close indeed).
In the low-end market, the price difference is also not very large, in the highend market the price difference is huge (often 50-75% cheaper for the Sigma or Tokina).
With Sigma, that would be the EX series. For Tokina, the ATX series.
I have one Tokina ATX-Pro lens, and I'm impressed with it. While the corresponding Nikkor would probably be better, it also costs 3 times as much (or 6 times if you take the 50% discount I got on the Tokina into account) and I doubt the Nikkor more than a few percentage points better.
While some people may want to pay $1000 just to get a lenscap and barrel engraved with the word "Nikon" or "Canon", I find that a bit of paint works wonders
and costs a lot less too.