What is your main reason for wanting a 70-300mm?
If it is for birding and other wildlife that is difficult to get close to you might want to consider getting something longer. I started with a 55-200mm, quickly upgraded that to a 70-300mm and I'm now looking to upgrade to either a 300mm prime and 1.4x teleconvertor or one of the 150-600mm lenses. I don't regret buying the 70-300mm as I couldn't afford better at the time and it was more important for me to be able to shoot wildlife right away. If you think in the long term you'll be saving for something longer or a lens of another focal length, I'd save the $250 and buy the Tamron, or just wait until you can afford the lens you really want.
I have been very pleased with my Tamron and have managed to get very good results with it, at least for me and my limited ability. I would be interested to try the Nikon version as some images I've seen on here taken at 300mm seem to look sharper than mine at 300mm. I don't know if that is down to the lens or just better technique and post processing from more experienced photographers than me. If you do buy the Tamron I can tell you that nearer to 250mm and f/8 to f/11 is where it performs best for me.
