I'm in the market for picking up a new AF zoom lens. I currently only own Nikkors, but I was wondering if I would be just as happy with a Sigma brand. I would like to stay under $200 and no more than 200mm or 300mm. Whenever I visit the local Wolf camera store, they usually try to push their in-house brand, Quantaray. I've been told to avoid those. What are your opinions?