i think the advice about photo tours vs. "sightseeing" tours - i've been on a couple "sightseeing" tours in various parts of the world and wouldn't consider doing that in Africa (and in general, 2 people can usually arrange a private tour with car and guide for very little more than a group tour most places and see more in a shorter period of time)
if you have 2 or 3 people it's not unreasonable to book a vehicle directly with the African tour company - i'd highly recommend Ranger Safaris in Tanzania and Kenya. They have the largest fleet of new and nearly new vehicles. In Ngorongoro and the central Serengeti this is important as vehicles must stay on the roads - with radio contact between many vehicles your odds of good sightings are improved.
unless you're spending more than 10 days, i'd recommend concentrating on Tanzania. between Tarangire, Ngorongoro, central Serengeti, and northern Serengeti you'll have a pretty good variety of conditions and wildlife. one thing to consider is that the restriction to roads in the central Serengiti is limiting - if you really want to get close a 24 hour photo pass costs $1000 per vehicle and allows off-road and night-time shooting - it takes about 2 weeks to obtain