I was planning to get the Nikon 70-200mm, but there is a big price difference compared to the Tamron. I will use this for sport shooting. Any advice?
The Tamron 70-200mm f/2.8 Di LD (IF) Macro lens is known to have fairly good image quality but very slow AF performance. Not exactly what you want to hear about a lens you are considering for sports/action photography. Here's the review of the lens at The-Digital-Picture.
The newly released Tamron 70-200mm f/2.8 SP Di VC is better optically and much better in terms of focus speed. It also costs twice as much as the older lens, but is still just under 2/3 the price of the latest Nikon 70-200mm f/2.8G IF-ED AF-S VR II.