I'd go to Yak and Yeti for dinner since you are already going to Boma. I've heard that Boma and Tusker House are very similar. But never been to Boma. Both are buffets.
Dinner at Tusker House in 2008 was wonderful. Buffet. Delicious food, lots of interesting flavors, big variety. Yak and Yeti for lunch in 2009, also great table service and delicious food. Menu has a variety of choices. I suggest looking at the menus. I'd love to eat at both! One for lunch and one for dinner. But last year we went to Jiko instead (also a great restaurant).
The best thing about Tusker House was getting to sample lots of different food. We also liked TH's breakfast with Donald Duck. The best thing about Yak and Yeti were the appetizers. The regular food was good too but I'd say that Tusker House had more adventurous flavors.