Do you have experience with SAA out of London also lie flat but on 744.
Better or not than Airbus out of FRA?
If you look into the SAA Website, they say, that at the moment the 744 will be refurbished, so it might happend that there is the old configuration with the older "Millennium Seats". I have flown them on CPT-LHR one year ago, very nice but fare behind the new seats.
So if you make sure to get the new seat, fly via FRA.