Prefer SAA personally
Well, I feel the opposite way to many here. I flew FRA-CPT on SAA and back on LH 18 months ago. Bot were overnighters. (I am very familiar with BA, Air France, Virgin, Continental, Delta, United, KLM and some other C-class setups, but not Singapore Airlines, for example.) Both flights were on A340-600s. The SAA business class seat was the most comfortable lie-flat I have ever slept on. You have a proper duvet and all. The entertainment system was OK, not fully interactive. The food was far better than on LH, cabin staff too.
It may have changed since then, but at the time, the business class in SAA 747s was not at all the same, and was like most airlines business class used to be 15 years ago.
At equal price, I would choose SAA.