Here is how we decided:
Boma is so expensive for dinner, but less so for breakfast. Sanaa can be significantly cheaper per person for dinner (depending, of course, on what you order). SO... we are doing Sanaa and Boma breakfast on our trip in July when we are paying OOP. For September, we are doing Boma dinner since we will be on the dining plan.
If cost is a factor and you are paying OOP, then I would pick Sanaa dinner or Boma breakfast. If you are on the dining plan, I would pick Boma dinner.
Oh- and one more thing: it might depend on how much time you have and where you are staying. From what I have read, Sanaa can take a long time - up to two hours. I imagine Boma would take less time since it is a buffet. Are you trying to get somewhere after dinner? How much time will you have? That might be a factor.