The Nikon version is far better than the Tamron. The sharpness is better. The focus is much faster.
I have the tamron and while it's a great lens, there is most definitely a reason why it's only $799 when the Nikon is $2400.
The tamron is noticeably slow to focus. For portraits and weddings it's great. For sports it's OK, but you will feel the focus lag at times.
Another option is the Sigma 70-200 f/2.8 and the Sigma 70-200 f/2.8 OS (OS is Sigma's version of VR/IS, etc.)
The original version with no OS on it is also slower to focus, but it is faster than the tamron. It runs about the same as the Tamron or a little more.
The Sigma OS version is not a macro lens and has the HSM motor-very quick to focus. Very sharp lens. Plus it has the OS (VR.) It runs about $1400
I have the Sigma OS version. I shoot with a girl who has the non OS version. She does shoot some sports with it and she says once in a while she misses focus because of the slowness of it, but not too awful bad. Her images are beautiful.
I absolutely LOVE the OS version. Crisp, clean, clear and I have no problem with the speed of focus at all. It's quick and silent. LOVE it. I use it for probably 90% of everything.