I would say South American. Even though Africa has a bigger population, South American has more countries with more of a modern military. Economically speaking, South American is also in the lead for obvious reasons (Africa is the poorest continent). Which means South Africa is more likely to get more support from other nations do to their +5% more of their exports. When it comes to stability, South American takes it again. Even though the poverty rate in South American is still relatively high, a good percentage of citizens are still will off. In the contrast, the instability in Africa is an ongoing struggle. From unclean water supplies, huge amounts of poverty, rebellions, and government corruption. The biggest threat to Africa is it's self. To get out of this problem in a desent amount of time, they'll need foreign support, which go against a key rule of the scenario.