I had both lenses and gave the Nikon to relative
Sharpness - not sure if I would agree that the Tamron is sharper from 200-300 . In all fairness I was comparing both to 70-200 ( 200 cropped in to be similar size was actually sharper across the frame than the 300 from either of these lenses ) . Both these lenses are sharper across the frame when stopped down a wee bit
VC - VC on the Tamron seemed to have better hit rate , but a bit distracting with the cluncks and viewfinder image "floating around" . This is unlike the Nikon VRI and II system which does final stabilization of elements right before shutter release. The Tamron is whirling away constantly
Autofocus speed - no, I would say both focus speeds ( from extreme ends ) was the same. Again, doesn't mean much for the simple reason is neither has as good focus speed as the 2.8 70-200 VRI or VRII . You notice when shooting moving people / kids etc .
Here is how you test . Depending on body, put AF-C priority on "focus " , and run camera at C high . You will know right away if lens autofocus can keep up with your camera . If you have priority on "release or release + focus" it will bang away at fastest frame rate regardless if in focus
I would rate both focus speeds at equally good but not great. Both faster than screw drive lenses
Build Quality - disagree Nikon build quality is better. Both are plastic body and feel the same
Price
At standard prices depends if you want Nikon brand or not. The vast majority of my lenses are Nikon , but the 70-300 is on of the lenses I feel aftermarked does a bettter job
Tamron often has rebates and when there is rebate its a total no brainer to buy the Tamron. I seem to recall I paid $350 ( $100 rebate ) from BH Photo