I love both places. It all depends on what you're in the mood for.
Jiko's is FINE DINING. It is, IMHO, the 2nd best meal on property (2nd only to Victoria and Alberts). Everything about the meal is amazing, and they have the most wonderful selection of South African wines. It IS relatively expensive, but worth every penny IMHO. If you're picky, you'll still be able to find stuff on the menu you'll enjoy (someone mentioned the filet on mac and cheese as one of the "safe" choices, for example).
Boma's is the best buffet I've ever eaten at...not just at WDW, but ANYWHERE. There is enough "traditional" stuff to satisfy the picky eaters and enough adventurous stuff for those that aren't. Everything, IMHO, is good: from the prime rib, to the "potatoes with affritude", to the wonderful mango vinagrette dressing, to the awesome soups (which change, frequently) to the crowd favorite zebra domes (dessert).....just talking about it is making me hungry.
You can't go wrong no matter which one you pick...it all depends on what you're looking for.