I had the 400 f/3.5 Ais, and also the 300F/4 with the Kenko 1.4. I did not like the 300 with the Kenko. Used both on my D3. I felt the IQ of the 400 F/3.5 was better. I heard good things about the 400 F/5.6. I am a pixel peeper, you will find that the lack of coatings on both the AiS lenses and the 300 ED will give you C/A and fringing....but for the price you cannot have everything.
Quite frankly, I did not like the 400 a whole lot.