A prime will give better results compared to a zoom. It will also be faster and lighter. If you do not plan to use TC17 or TC20, then the older 300mm F4 D AFS is a better option. Reviews state that there may be QA problems, so the lens is not as sharp at edges as the old one, and some report VR issues. The major advantage of the newer 300mm F4 is VR and its lack of weight. If these are not your priority, then the older lens is a better buy, as you will get the lens and TC14 for less than the new lens only.
Thanks aroy,i had a feeling i knew which you would recommend