I had some remarkable experiences in Mozambique, and equally remarkable ones in Namibia. They're very different--in culture, colonial histories and remnants, climate (and therefore wildlife and vegetation), geology, infrastructure, politics...

So this really depends on your interests. For wide open spaces, deserts, hints of ancient populations, desolate seacoast, and pretty good travel infrastructure Namibia seemed like the promised land after roaming around East and Central Africa.

Mozambique was more about tropical beaches (with outstanding diving, by the way), recent colonial ruination as well as slave trade relics, and--at least when I was there--the experience of harsh travel in areas where other tourists had never been. The absence of much infrastructure meant we ended up hitchhiking on private boats, freight trains, and one private plane, in addition to taking some remarkably long and uncomfortable bus rides. I know less about what it might be like these days or the extent of rebel activity in the north, so take my descriptions with a large grain of salt.

A month would be an appropriate amount of time for one or two of these countries, depending on how much you like flitting around. I'll add that there are also questions of budget, since (for example) Botswana trends quite expensive, and costs in Namibia can add up quickly too due to the distances and need for a rental car.

I'll stop now. Hope that's helpful!

Mark