They are both great, and if the amount of credits you have to spend on meal doesn't matter, then decide on what type of dining experience you would like to have.
Boma, while a buffet, is fantastic. You can still have an african beer or a wonderful african wine with your dinner. The time needed will be a good hour, at least for us it is anyway, lol.
Jiko is a fine dining experience, plan a good 2 hours at least. You are not rushed here, time between courses to enjoy. The food and service are oustanding.
We love Boma, but prefer to eat at Jiko.