This is discussed in my upcoming review. But, briefly, I tested the Tamron against the equivalent Nikon and it turned out to be slightly better lens. This is corroborated by reviews that I've seen online.
70-300mm is a favorite of mine for urban shooting and travel, but frankly none of the offerings are great. But, better the lens you have than the one you wish you had.
BTW, I have two Sigmas for use with the new Nikon bodies and they are both superb, the 150mm f/2.8 APO Macro, and the 120-300mm f/2.8. More on these in my upcoming review.
Michael