Antimalarials a must!
A backpack is by far the best option, unless you are planning to do an organized trip and then either will do - as long as it is light....
It can get very very hot, at any time of the year, so light breathable clothes, but also take a jumper just in case, because the nights - particularly in the Kalahari can get quite cold at any time of year.
Your own needles and also anti-malarials and insect repellant.
Away from the tourist centres, it is very difficult to get the film or special camera batteries that you need, so it is probably best to buy in the cities or bring it from elsewhere.
A waterproof jacket will come in handy for keeping you both warm and dry, should you need either.
A torch is invaluable, particularly in rural areas.