Each season has its advantages and disadvantages: If you go in the dry winter months (May to October) animals are congregated around waterholes. Early summer is the time antelopes give birth, so there are lots of predators. In the rainy period of summer, January to April, there's a lot of migration through the Kalahari.
here's a guide to game viewing in Botswana from WorldNomads.com:
and an all Africa safari guide
Phil at the safety hub about 5 years ago
Namibia high summer (December-January) will be too hot. Spring, winter and autumn are great. Phil has good advice. Winter is nice because not only are animals near waterholes, but there is less foliage so it is easier to see the animals. Afternoon weather in winter is balmy (25-30 Celsius), but dress for the chilly nights and early mornings (usually 5-10).
Here are some essentials to bring with you for a safari in Namibia: http://traveleater.wordpress.com/five-essentials-for-a-safari-in-namibia/ about 5 years ago