It depends partly on where you're going. Serengeti and other parks, you're a reasonable distance from the wildlife. The nice thing about the 300 + 2x is that you have the 2x if you need it, but 300 is a good length, and f/2.8 is good when the wildlife is active (dawn/dusk).

If you go to Ngorongoro, the 24-105 will be fine much of the time.