What lens to choose for DSLR camera?

I have a Nikon D3000, which needs a motor in it's lenses in other to be able to auto focus, I have looked at these two lenses but can't decide which one to buy.

http://www.amazon.com/...0012X43P2/

http://www.amazon.com/...0012UUP02/

What is the difference between the lenses and which one should I pick and why?

On paper the only difference is the price, which favours Tamron. Reviews and comparisons suggest that the Tamron is sharper at 200-300mm. But is more prone to Chromatic Aberration (colour fringing). There have also been some complaints about paint peeling on the Sigma.

I have 10-24, mm 17-40mm 17-270mm, 70-200mm, 150-500mm plus a series of prime lenses in addition to lenses for my old film cameras. These lenses include Canon, Tamron, Sigma, Zeiss (Ihon and Domiplan) and while there are real differences between lenses, the reality is that I have never regretted owning the Tamron or the Sigma lenses. I doubt that anyone will ever be able to tell whether your photos were taken with a Tamron or a Sigma lens!