FALA is probably the best for bizjet arrivals, FAJS is airline territory.
FALA - FAJS by road is maybe 30 minutes, mostly on the freeway or you could use a helicopter transfer if your pax are feeling rich.
You will be messed around for at least 30 minutes at FAJS immigration so heads you win, tails you lose.
OTOH, coming from Oz it would be much simpler to avoid Joburg and go direct to FACT.