For me - its an easy answer to give ---- JIKO!!
I have done both - in fact, on my last trip (Sept 2001) we did Jiko first and loved it so much, we decided to return the next night for Boma -- what a disappointment!
Jiko --- fancy, romantic restaurant. No doubt a little pricey but all the restaurants in Disney are pricey. We had an amazing appetizer of these rolled meat cigars (don't know what they are called but our waiter recommended them) and I had the filet and my husband had a paypapa that was roasted with the same meat from the appetizers. One of the best meals we ever had --- and a giant stawberry shortcake that we split for dessert.
Boma --- a buffet so much more relaxed. If you are not an adventurous eater you will not like this place. The meat was cold and tough and the food choices were so atypical I had a hard time finding something I liked. For example, you go to make a salad - easy enough - but all of the dressings are not the type you would normally find (things like curry and lemon vingegrette) Anyway, I just did not enjoy it.
I am always amazed reading all the reviews on this board that rate Boma higher than Jiko. For my money, you can't beat Jiko - the food, the atmoshere, the waitstaff, the fancy factor...it is truly a top notch disney dining experience.
My husband and I just recently picked the rest of the restuarnats we would be dining in on our trip this September -- Jiko was in place however since the trip idea first originated - about 8 months ago!
We are dying to return to Jiko and I suggested that we just eat there several nights during our trip - but my more adventurous husband wants to try other places as well. I am dreaming about the cigar shaped meat appetizers (wish I could think of their name) and me and my husband have decided we are not going to split one this year - we are going to each get our own! They are that good.
So - in conclusion - sorry for the length of this post -- my opinion, (if you can't try both and have to pick) is JIKO!!!!
Have fun!