I would vote for Tusker House. It is so nice to be able to take a break after morning touring, convienent too. We have been to Tusker House for breakfast and it was good. My friend said she really enjoyed lunch there.
Boma is nice, although we won't go back. It's one of those things that we are glad we tried but don't need to do again kind of things. It just seems like such a process to get there if your not staying at AKL. You will definitely need some time doing Boma. Just My humble opinion.