Like everyone has said, there is no comparison. They are two of the best restaurants on disney property.
Boma, buffet, it will probably take you about an hour to eat there.
Jiko, upscale, New African cuisine.
Food is excellent at both. I would do both, and we have done both in the same weekend, Boma for breakfast and dinner, Jiko for dinner and room service in that mix. It was fantastic and we never felt like we had the same meal twice.