Just to play devil's advocate, why a prime? You're going to be on the sidelines of a sport where the players are moving up and down the field, so the action might be 15 feet away, or might be 50 yards away. To me, that situation calls for the flexibility of a zoom like the 100-400mm.


However, if you're going out with the 70-200 f/4 on a second body then a prime would make sense. You stated, "I have a Kenko 1.4xtc that I use on the 70-200 and get great results at 280mm but find the need for a longer reach most times." If you really mean most times, why are you considering the 300mm f/4? If you'd said your problem with the 70-200+1.4x is that at 280mm f/5.6 is too slow, or IQ is inadequate, then the 300mm f/4 would be the obvious choice. But 300mm is not a 'longer reach' than 280mm, IMO, so if you're going with a prime you'd want the 400mm f/5.6. But then I'd come back to the 100-400mm, since at 400mm it's no slower than the prime, and offers much more flexibility, IS just in case, and only a very minor hit on IQ.