Buy the sigma 105 vr instead and save yourself a fortune.
It begs the question, what's wrong with your Tamron?
I had the latest 90mm Tamron for a very brief amount of time and as far as I can recall it was better than the Sigma.
Between the Sigma and the Nikon I like the Nikon more but it's quite prone to CA, it's not as sharp as the alternatives and it's quite a bit more expensive.
Im not sure the Tamron IS the latest version either.....
You say the Nikoor isnt as sharp as the alternatives - which are ?