.257 Roberts
Because I'm guessing that 7x57 Mauser brass will be easier to come by in a country with European, rather than American, hunting traditions.
Lots of military Mausers have been converted, and if you can't find one, the conversion is just a barrel and a gunsmith away.
Let's not make recommendations based on ammo availability in America. The OPs situation is likely to be very different. Also, I don't know if the OPs country restricts military calibers, but many do. If they do, having a cartridge that uses military brass but is not a military caliber may be a significant advantage.
The .243 is based off the .308, and the .270 from the .30-06, of course. I'd think, based only on my limited knowledge of armies and what they used, that of those two, the .243 (or any other .308-based cartridge) might be easier to feed. I'm pretty sure the SA Army used the 7.62x51 for a long time. The .30-06, not so much.
But I'm not sure the .243 is enough gun. I've never been to Africa, so I might well be wrong about that.
--Shannon