Nikon Lens vs Tamron / Sigma

I'm happy to use any brand of lens and generally do my research before buying. I recently sold my Nikon 16-85mm VR due to exposure issues (could be as much as 2-3 stops out) and decided to buy the Tamron 17-50mm 2.8 (non VC) and Tamron 28-75mm 2.8 for the price I sold my 16-85mm VR. I still regularly shoot film so I mainly use the 28-75mm on my F100 but I find it works brilliant when I use it with extension tubes to shoot macros on my D7000. I also own a Sigma 10-20mm f4/5.6, Sigma 24mm 2.8 Super wide II and a Tamron 70-300mm VC. The only Nikon lenses I own currently are a Nikon 35mm 1.8G and 50mm 1.8D.

I generally find from my own experiences the Nikon lens have better build quality and can perform better than third party lenses. However, optically I find there is little difference if any from Nikon's equivalent line of lenses. In fact, I would say my Tamron 17-50mm 2.8 outperformed my Nikon 16-85mm in image quality.

If your really unsure pop to a camera store that stocks both lenses and try them out for youself.

One thing I would say is if I had unlimited money I would probably own all Nikon lenses however as photography is just a hobby for me, I go with whatever best overall value lens that are available at the time.

Hope this helps.