If you were only doing one I'd say Boma. Being a buffet you can sample a lot of different flavors and then go back to the ones you like if you aren't full. If you are a coffee drinker their press pot is the best coffee on property IMO, even better then the Kona at the Poly.
They are both excellent. I agree that for the food, if you're only doing one, you probably want to do Boma. My son much prefers the food at Boma. However, the view from Sanaa is much better, particularly for children, since the windows look out over the savannah.
This is a tough one. Having eaten several times at both restaurants, I'd have to say Sanaa. It's actually my absolute favorite 1 TS restaurant on property. It's SO good. Not only is the food excellent, but the restaurant is beautiful and whimsically designed AND you get to watch the animals walk by just outside as you dine. Just an awesome place overall!
Sanaa is one of our favorite restaurants at WDW. Where else can you eat and have animals randomly come up a few feet away? The food is great. We like Boma (ate there twice on our last trip) especially for breakfast, but Sanaa is truly special.