thank you so much for the replies...
I finally stayed in Rosebank (near the Rosebank Mall where there's an African Art Market on Sundays on the rooftop). I also visited Sandton for dinner one night, I managed to visit Soweto (Mandela House, Hector Pieterson museum, Freedom square. Half a day to do the visit) and the Apartheid museum (you need about 3 hours to visit the museum. A very moving experience).
In Pretoria I only got to visit the Zoo (drove by downtown and the Union building but did not visit). The zoo was great.
The Gautrain was very convenient and inexpensive....
Most of all, the south africans of all races are wonderful (but still segregated I have the impression, compared to France for instance).
Flew the AF
A380 and I must say their seat in Y was not comfortable (could not sleep more than one or two hours)!