I am also not a fan of the Club World seat. Yes, it is private - but it is cramped and there's nowhere convenient to put anything. I much prefer the SA seat.
Having said that there are times I will favour BA over SA in J:
- if I am flying LHR to/from CPT and can get a direct flight (almost anything beats connecting in JNB)
- if I can get 64A/K on a 744 - in fact any window seat on the UD would also do
The BA lounges in JNB are better than those of SA IME. (The food offering in the SA international J lounge in JNB is a disgrace!) Plus they have arrivals lounges in LHR. But OTOH SA has an arrivals lounge in JNB. Leaving LHR I like the Maple Leaf lounge that SA uses and I think it's nicer than the Galleries Club lounges.
IMO food/drink on BA is very mediocre and I regard SA's offering as a step up.
Service is always a variable on any airline and both BA and SA can be good and not so good - but SA is far more likely to deliver atrocious service than is BA. You know, the sort of service where the crew is belligerent, surly or just disinterested and make up the rules to suit themselves as they go.