Originally Posted by
Manfred M
The Nikon lens is well regarded in the marketplace as a quality lens. The main reason that most people will use a third party lens has generally been either a lower price or having features not available with the OEM brand.
The main downside of Sigma is that their controls (focus and zoom) run in the opposite direction as native Nikon lenses. Tokina works the same as Nikon, but once one gets used to it, not a big deal.
Neither the Nikon nor Sigma lenses are particularly "fast", i.e. smaller maximum aperture lenses. This may or may not be important to you. If you are looking at doing night photography, that could be a deal breaker, but if you are looking at landscape or similar work in daytime conditions, it would not matter as much. That decision generally results in a lighter and less expensive lens.
When I bought my Tokina, one of the deciding factors was low light performance, so the extra 2/3 of a stop is shooting speed was the key decision in buying that lens.