Rusty and Lizette have pointed out the features separating this bird as a Black Stork already but I just thought I'd add that the legs on Abdim's Stork almost always (even in young birds) show a two tone colouration with the tibiotarsal joint and the foot being more red than the rest of the legs. This bird clearly appears to have uniformly coloured legs.
But like the others have said, the size of the bill and just the general shape of the bird points strongly towards Black Stork already.
As for their relative abundance I could add just this perhaps. Black Stork is a resident bird but fairly uncommon and, although widespread, perhaps a bit sparsely populated. Abdim's Stork on the other hand is a non-breeding migrant but when they are here they are often present in good numbers making them seem more common than Black Stork. The fact that they're highly gregarious also gives the impression that they're very common.
That said, I think Black Stork is quite readily found in Kruger and the lowveld while at the same time quite rare on the highveld here around Gauteng and the Free State. Abdim's Stork on the other hand is a common bird around here so you will probably find that Vaalies like me get much more excited when seeing a Black Stork than Abdims while very regular visitors to Kruger might have a different view, although jacliz, you've probably just been unlucky not to have ever seen Abdim's in Kruger.
One last interesting comment about Black Stork: They breed widely throughout the Palaearctic region and migrate south to southern Asia and sub-Saharan Africa. However, the birds that occur in southern Africa are not migratory or at best regional migrants, moving not much further north than Zambia and Angola. In other words, ours is an isolated population that do not come into contact with the vast numbers of Black Storks in Europe and the northern parts of sub-Saharan Africa. So watch this space fellow arm-chair tickers (if you're prepared to wait a bit), in another couple of million years they may even evolve into a separate species