Then South West Africa, Namibia was a very quiet and little-known backwater when I first hunted there in 1979. Today, this country hosts Africa's second-largest hunting industry, and I believe it now holds ten times the game than when I first hunted there. Conditions are generally the same as South Africa: modest daily rate plus trophy fees for game taken, with packages often available. On the plus side, the average property is larger than South Africa, and game fences, though rapidly proliferating, are not yet endemic.
On the downside, well, there really isn't one. Namibia is generally an arid country and does not have the extensive species list of South Africa. On the other hand, game ranching has similarly moved things around, and in any case, on a first safari, what does it matter whether the game list is 12 or 20 species deep? You aren't going to get them all on the first trip anyway. One salient point: Although Namibia also has a very competitive safari industry, a basic plains game safari there is generally a little bit more costly than South Africa.
Infrastructure is similarly excellent, with good road networks, but both are big countries, so you can eat quite a bit of time if you want to move around. My recommendation for a first safari, in either country, is to pick one area that has a selection of game that appeals to you and stick with it. On later safaris you may want to move around and hunt more specialized animals, but on that first plains game safari, stay in one area, hunt it properly, and enjoy it.