If you were only going to one, which would you choose?
And what are your favorite entrees at either one?
The answer:
It depends.
I love both places...but for different reasons.
Jiko is a fine dining establishment.....so if that's what you're looking for, go for it. The food is out of this world (though pricey), and is meant to be enjoyed. It's also, IMHO, a nice romantic spot for a night out with the significant other, or to celebrate a special occasion.
Boma's is the best buffet I've ever eaten at....but, IMHO, the food is not as good as it is a Jiko's (but then, IMHO, the only place on Disney property that's any better than Jiko's is Victoria and Alberts). Then again, the atmosphere is a lot more laid back and the prices are a lot lower, too.

There's LOTS of variety, which is just a wonderful excuse to try lots of stuff.
We try to do both....but when push comes to shove....we end up choosing Bomas. That speaks more to how family friendly we find Boma's to be (not that Jiko's NOT....just less so, IMHO), rather than anything else. If you asked me to simply choose my favorite, I'd say Jikos.