Hi Helen,
We have done Boma for Thanksiving and it was great in terms of food options and very convenient (we always stay at Kidani). We have done Boma, Sanaa and Jiko. I would say Boma was the best as far as food choices (Traditional Thanksgiving, plus all the regular Boma choices).
My only issue with Boma was the buffet lines were rather long and we spent a lot of time apart while getting food. We have found we like being served our meal for this reason. If it will not bother you, go for it!
We will be doing Sanaa again this year. After many years of Thanksgiving dining, it has been our favorite for food quality, atmosphere, location, and service! It will be our first repeat!