Any more info regarding the OP's question? I'm Plat and am trying to narrow down a property now that the SPG option is fully booked.
You may want to look through this thread, I think there's quite a bit of discussion of Johannesburg hotels in there:
The only JNB hotel I stayed at was the Protea just outside the airport. That was fine, but I wasn't sightseeing Johannesburg, I was in between a incoming domestic flight and an international flight the next morning (and thus didn't qualify for the Protea Transit).