I have done both.
While I agree with the consensus that, generally, 'it depends', I would say that if I knew then what I know now, Namibia would win as a first timer to Africa hunting destination. There are a few of reasons for this, but the main ones are that the hunting areas tend to be enormous, and there seems to be a bit of a wilder flavour to the place than you will generally find in South Africa.
Minor reasons would include such things as not having to fill out a long form to get rifles in (the Namibian form is one page, you fill it in on arrival, and it takes minutes to get the guns in), and a generally more congenial atmosphere with respect to racial issues - the government in Namibia doesn't seem to sing from the same divisive song book as does the government of South Africa. Equally, I think that Nambia has virtually every animal which South Africa has, that a first-timer would be looking at, often at lower trophy fees.
Before I get drawn and quartered, let me be clear. There are large hunting areas in South Africa, and there are some which seem pretty wild. But it's a fact that South Africa is (much) more developed, and has (many) more people, than does Namibia. And there are game ranches in South Africa with very reasonable fees, comparable to Namibia. I am not being critical of South Africa, and in fact, am likely back there this winter for the last two of the tiny ten I am looking for. It's great hunting destination, but you asked for a view, so I've given you one. Suggesting one doesn't mean denigrating the other (do I sound defensive yet?).
I love both places as hunting destinations, but on balance, I think Namibia might just have a little bit more to offer a first-timer.