of the 2 choices you have mentioned, I would suggest the 400f2.8 L V2 + 1.4, I have & use this combination for Safari Photography & find the 400 an amazingly sharp & fast Lens. The 500 I have looked at recently but went for the 600f/4 L V2, again as it's the preferred choice for Safari work. I would also mention that the 300f/2.8 L V2 is my most used Lens (other than the 70-200f/2.8 L II), the 300 is without a doubt the fastest and sharpest Lens Canon make. The 200-400f/4 (1.4x) has been mentioned and I would agree that this would be a possible alternative for your needs, you give up some light (f/4 compared to the 400 @ f/2.8), but the biggest issue for now is the Lens doesn't exist in anything but a prototype form, I am not sure how long now Canon have been hanging this Lens out there as something about to be released, but it's become a bit of a "Never Ending Story", which is a pity as it's certainly a Lens that I would Buy & use.
So, 400f/2.8 L V2 + 1.4x, Unless Canon release the 200-400 tomorrow.