I have taken elephant, hippo, and buffalo (2 each) with both a .375 and a .458 plus one lion with the .375. If I ever go after such dangerous game again, I will take only the .375. I will probably not take a backup rifle.
If I go again for plains game only I will probably take my .300 Wby again, though I am thinking it would be nice to hunt some with a 6.5x54 MS model of 1903 with iron sights. A 30-06 or equivalent would be adequate for all plains game, IMHO, and also for leopard.
I prefer bolt actions with moderate powered scopes. It is often difficult or impossible to mount a scope on a double or to maintain regulation with one. Some doubles are dangerous because they can discharge if you drop them on the buttstock. Besides, my "muscle memory" makes operating a bolt routine.
If I go again for plains game only I will probably take my .300 Wby again, though I am thinking it would be nice to hunt some with a 6.5x54 MS model of 1903 with iron sights.