They are both wonderful. You have to choose based on what you are in the mood for experience wise and how much time you have and if you have children how well behaved are they.
Boma, a buffet, but relaxed, no hurry, take your time and try everything on the buffet..lol. Plan a good hour to 90 minutes for this. You can get a nice african wine or beer with your meal. Very enjoyable.
Jiko, upscale, time between the courses to digest, wonderful wine list, outstanding food and service. One of the top restaurants on disney property. Plan a good two hours or more for this meal, mostly adults, so kids would need to be good for that lengthy period of time. Now, there are always kids when we dine there, but they are very very well behaved.