I haven't been to Africa yet, but out of what I've read and heard from other experienced hunters this is my choice:

Merkel .470NE open sights
Tikka .30-06 Zeiss V 2,5-10x50

I've long wanted a 9,3x74R or 8x57JRS double, for which I'll mount a 1,5-6x42 Zeiss with a german claw mount.. When I get this one, I'll skip the lesser bolt gun altogheter....!

I like to keep things simple. You're not likely to walk around with all your rifles at the same time? Chances are pretty good that you'll have the wrong gun in the right moment..

I'd like to make a combined hunt for buff and plainsgame when I find an opportunity to go to africa. There are no plainsgame that will recover from a well placed .30-06 bullet. When it comes to the big stuff I can't see the need of a scoped bolt gun unless because of cost? I like to get up close, not shoot at long distances-that's marksmanship, not hunting in my world... And as long as eyesight is good, a shot at 50m at big game shouldn't be a problem with a double with open sigts. With the big stuff I've heard bigger is better, so I rule out the scoped medium big bore.

Only one rifle? Those who choose .375 for everything can't go wrong!