Can't go wrong either way. Some folks think of Tusker as "Boma Junior", but the Tusker folks themselves think of it as "Boma Plus". As a buffet, they have a great selection of both African themes and "everyday" food - you can sample a bit of everything, while at Yak & Yeti you're ordering one dish.
Both also have interesting decorations.
If it matters, I find I have always had to wait a while at Y&Y, even though I have an ADR and I go in the slower seasons.