Given the opportunity, the .458 with a smaller plains-game rifle in .375 would be my choice.
: Since shooting and hunting with .375's, H&H and '06IMP's, albeit in North Amercia, I'd have no trouble with it's use on all African plains game. Overkill on some, and perfect for others.
: If confined to one gun, it would be a .375H&H with premium bullets, of course, instead of the .458. I am quite comfortable with a .458 for quite long range, however the .375 certainly makes for less guess work. I do know that a .375 seems to anchor out larger Moose just a bit faster than the .458 with the same chest cavity hits.