I can also jump in here a min to give you a small bit from the Chef's themselves. We took the culinary tour so this may help you out.
Boma - Great place to try lots and lots of different things. Fish was awesome here, very tender, melted in your mouth and had wonderful flavors.
Boma is where they really try to capture the African tastes, they try to cover several of the many countries of Africa and you'll find a wide range of flavors.
Like others have said Boma is more upbeat and casual. Our service was fantastic and since we went later it wasn't real crowded and so it wasn't too loud. A very relaxing experience.
Jiko - We wanted to try this place, but with our youngest in tow this trip we figured we'd save it for a future one, I don't think Jiko is going anywhere anytime soon!! I did get to sample some items there on our tour and if they are an example of Jiko you won't be disappointed.
This place is very beautiful and very different in decor then Boma is. It is very quiet and more on the romantic side, not like Cali Grill or V&A's but right up there. It is a darker restaurant, but I didn't think it was overly dark, fits the mood I think.
They have a wonderful open kitchen that they use to prepare some of the menu items. As to their menu and foods, they are not an African dining experince, as many folks think. The chef said they use an small bit of African influence, but they also take from several other cultures, as well. At times the smells coming from this place were so heavenly.
I don't think either place would be a bad choice, guess it comes down to what you want from the experience. Good luck, this is a tough one!
Pammy