Africa Adventure Company. Mark Nolting's (the owner) book Africa's Top Wildlife Countries is an outstanding guide. We've used Africa Adventure company for South Africa, Botswana, Namibia, Tanzania, Rwanda, and Madagascar.
The advantages of a private tour:
if you book with a good agent, you will have the same drivers and guides as the photo safaris - and these guides will be outstanding
you won't be paying for the pro photographer(s)
the guide will consider your particular interests
with only 2 in the group there will be lots of room to shoot from the optimum location
the tour itself will be designed for your schedule and interests - AAC found a camp for us in Botswana that gave me an Aardvark sighting
the advantage of a photographic tour
if you don't know what you are doing you will get help - but it will be the most expensive photography lesson you can imagine