Just got back from India Safari - Gir Forest National Park (Oct 2011).In India they use little Jeeps (Suzuki/Maruti) that can seat 4-5 people and are open top.
Unlike East African Savannah safari, due to the dense Jungle nature of Gir forest just after the Mansoon period I found it would be better to take a zoom (Canon 100-400L or in my case Canon 70-300L) because anything further then 20-30m would be hidden beyond view and also you have better versatility. However a friend of mine wished he had a 1.4x + 600mm for a tiger in Jim Corbett NP once. The point is - if you can afford it take a range of lens.
In Africa (Tanzania / Kenya) reach is everything - I have used the Sigma 100-300 and the Canon 100-400L both on bean-bags. However I also found that spending all day moving a big heavy lens around the car does tire the arms.
Because of this and cost reasons my primary telephoto lens is the new Canon 70-300L for use both on Safari and at home at parks in the UK with my family.
Beautiful photo of a rare lioness.
Beautiful Photo, out of interest where in Africa was it taken? Also I am debating between the 100-400 and 500 F4 you have used for safari - any recommendations.