The 300 f/2.8 with TCs is much sharper than the 100-400mm. If you are going on a bird shoot on Safari, then I would definitely prefer the prime. But, my experience from my one safari is that the zoom would be far more useful for the wild life as the game wardens take you very close to the big game and the 300 would be useless. (I have both sets of lens and would pack the 100-400, which retracts nicely to a small size too). On my trip, I had just the 70-200mm f/4 with a 7D, and that was fine for most of the time though double that would have been useful for some of the time. Next time, it will be the 100-400 for convenience.