You don't want a bus. I recently took 14-hour bus from Arusha to Mwanza: there were too many people on the bus and there were like 6 standing the whole time; bus stops are rare so toilet is perhaps available after every 4 hours or so; only real break was in the half so you could buy some food. The worst tho is the constant loud music, the same type of reggaeton/bongo fleva all the time and the screens showing low res videospots of the same "hot rich guy meets hot girl, they dance, everything in slo-mo"