I have a D300 and both the 70-300 VR and 70-200 VR f/2.8 with TC 1.4. My main subject matter are airplanes at air shows.
For image quality, you can't beat a good copy of the 70-200 f/2.8. Add the TC 1.4 and you still have a constant f/4. With the 70-300 you have f/4.5 to f/5.6.
Advantages of the 70-300 is that it is more compact, lighter, a whole lot less expensive, and easier to take along on trips.
Advantages of the 70-200 & TC 1.4 combo is that you can achieve f/2.8 with just the 70-200 along. Disadvantages are that it is much larger, much heavier, costs about 3-4 times as much and can be a b*tch to take on trips. Also I have experienced the dreaded DBS syndrome 3 times with that combo, during an air show shoot, which caused me to miss important shots.
If money isn't an object then do what I did, get both. I use the 70-300 as a general purpose carry around lens. I then use the 70-200 combo for events where I know I need the extra speed of that lens and extra image quality.
You can see samples of my photographs with both lens at my Smugmug gallery.
Now if I could just afford to get the 200-400 VR........
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Clifford W Martin
aka BigRedDogATL
http://cwmartin.smugmug.com