Jiko is perhaps, my favorite overall restaurant at WDW. Very relaxed ambinance and consistently good food.
Boma, on the other hand, is arguably the best buffet at WDW. Even though it's not a cheap buffet (even by Disney standards), it still provides more bang for the buck than Jiko from strictly a food quantity and selection standpoint (my favorite thing is the ribs, which are only served on Wednesday and Saturday).
If I had to choose between the two, and money were no object, I'd probably pick Jiko because, as a signature restaurant, the food quality is higher (even though Boma's quality is very good for a buffet) and I prefer a full-service sit down dinner over a buffet (the overall dining experience for a full-service sit down meal is just so much more relaxing). Most people under age 30, however, would probably prefer Boma.