If you have to chose just one I would definitely eat there at dinner. There are more exotic items at their dinner. We loved the dinner there so much that we went back from breakfast on our last day. The breakfast was really good, but they had more diverse and interesting food at dinner.
For breakfast they had the traditional scrambled eggs, create your own omelet, pancakes, waffles, french toast, cereals, pastries like pecan rolls and muffins, bagels, sausage, ham, and mixed fruit among other things. The exotic dishes they had were grilled tomatoes, sweet stewed tomatoes, turkey (not usually served at American breakfast), a curry sausage biscuit dish, bobotie (like a spiced meat bread pudding), and some of the sauces they serve at night as well. If you go for breakfast the coffee is wonderful!
For dinner they had about four or five soups each tasty and uniquely spiced, a dinner bobotie, sweet stewed tomatoes, about 8 different interesting cold dishes, sweet potatoe dish, spiced beef and pork, four different sauces, roasted chicken, couscous, flavorful rice dishes, cornbread, and three different types of hummus with flatbread and pita. (This is just what I remember there was more!) The deserts were really good as well. They had a large selection of mini desserts which are great for those who like to try a lot of different things - zebra domes, carmel cheesecake, cinnamon covered chocolate mousse, and peanut butter bites, and cookies just to name a few.
Our best meal on our trip was Boma's dinner buffet. I only wish we had a restaurant at home that served African food let alone the quality we got at Boma. Enjoy your trip, and I hope you enjoy your meal at Boma!