Need your opinions about lens

I am looking to add a 300mm lens.  Saw a Nikkor 300mm F2.8 AF-s for only $1000 on local craiglist and I was the first caller.  Waiting two hours after work and someone else got it before me.

Now there are two options - a Nikkor 300mm Af-s F4 for less than $900 or a Tamron 300m F2.8 for about $1200.  The sample photo from the Tamron owner was not very sharp and I am concerned of focusing speed.  The Nikkor, on the other hand, has a pre-2005 serial #. 

I would really appreciate those who has experiences with the Nikkor. 

I do have a TC-17E II.  But the focusing is very slow when I use it on the 70-200mm VR II.