It just depends how thin a depth of field you want and how much room you have in your pack/how much weight you want to carry? The 2.8 is like 75% heavier and takes up way more room in your bag. In both cases your shooting in sunny conditions so you won't need the fast aperture, and the F4L still has great bokeh wide open. If you can afford it I'd go F4L IS for photography, again for video IS is pointless cus you'll have it on a tripod, it can be nice for photography, what percentage do you shoot each? If you can't drop $1000 on a lens, the F4L non-IS is a great value, its just better to have it on a tripod (not sure if that would work on a safari).
You might also want to look into some extenders for your safari, they can be really helpful. That is also a factor on which lens you choose, I believe the 2.8 handles them better because of a change in your effective aperture, certain cameras don't have auto-focus for higher native F stop lenses. What body do you shoot on? Check out Dan Carr's blog for help on that, he uses them a ton on his F4L IS with his 1d's.
I'd just stick to the Canon versions. Sigma makes some good glass, but Canon is renowned for its 70-200 lineup, some of their best lenses.