If I were you, I would go with Boma. Both restaurants are fantastic, but Boma has some more familiar, simple things on the menu, like chicken fingers, fries, and cookies for dessert. Then, you could try anything else you want that is different from what you normally eat and decide if you like it. If not, you have the "safer" more familiar items to eat.
Sanaa is excellent too, but you are more limited if you are a simple eater. They have a regular grilled NY strip steak, but their other food seems to be more on the exotic side. That's right up our alley, which is probably why we loved Sanaa so much when we ate there last month. However, if we had been traveling with other people who were not into exotic and different tastes, we probably would go with Boma. My kids loved Sanaa because they have a kids' menu with usual kid fare (pizza, etc.), and the view of the animals (we were at a table by the window).
