I've eaten at both and personally like Sanaa the best. But for non-adventerous eaters I would recommend Boma. Since it's a buffet you don't have to 'commit' to the food like you would at Sanaa. If you don't like something at Boma, you can simply go try another dish!
If there's anything on the menu at Sanaa that sounds appealing, I would encourage you to try it. Things are not spicy (as in hot) there. But you can ask they be made spicier if you like! I like the ambiance at Sanaa better - it's more subdued and not so boisterous. Plus, the view can't be beat!
Here's a review I did of Boma:
http://www.disboards.com/showthread.php?p=35847358#post35847358
And I haven't done a review yet of Sanaa, so here are pictures:
A look inside:
Salad Sampler - Roasted Beet Salad, Cucumber Tomato and Chickpea, and Watermelon, Lime, and Radish. - All Excellent!
Slow Cooked in Gravy, Simple and Well Seasoned - Red Curry Chicken, 5 Grain Pilaf, and Beef Short Ribs - Loved all these too!
Dessert Sampler - Chocolate Cake, Chai Creme, Passion Fruit Kulfi - Cake was dry, but the other two were really yummy!