We love all three of these restaurants, but as previous posters have said, they're pretty different.
I think the food at Boma, while I love it!, is a bit more "for the masses," as in less spiced, milder, etc. Sanaa has a strong Indian influence, so if you don't like Indian food, I'd skip it. One of the things I like about Jiko is that the chefs are always experimenting with recipes, so there's something new every time you go. The tandoori chicken is the best chicken I've ever had!
To the above poster who said Jiko's food was mediocre and service awful, I'd urge you to try it again. We've always had wonderful food at Jiko, and almost always had wonderful service. A couple of times our servers have let us down, but for the most part it's been great.
I'd say check out the menus (but remember that Jiko's changes seasonally, tho certain items like the filet are always there) and see if it sounds good.
