I flew both DL and SAA to JNB. I preferred the service and the seats on SA. The seats on SA were lie flat (totally, not at an angle), the wine selection was better, the flight attendants were ever present, kept refilling wine and water glass. The duvet was nicer quality than the DL one. Breakfast was also better. When I got to JNB I was sooo relaxed and ready for a vacation.
However, the DL flight was also good, even for the long duration. Your standard Business Elite, better than UA.