Plusses for LHR:
Daily frequencies (vs. 3 per week from FRA)
Own arrivals lounge in LHR
Own departure lounge at LHR (but it really isn't wonderful apart from the showers; pathetic snack and drink choices and often surly staff)
Choice of C or F
Plusses for FRA:
A340-600 with great lie-flat beds with hard shell (though the new seats in C on the 747 are also very nice)
Not sure what the LH lounge used by SA is like
The arrivals lounge at LHR is very adequate and has a most congenial ambience. Very nice, spacious showers; a selection of hot and cold meal items; internet access etc. Since all the flights from CPT and JNB arrive in pretty quick succession it does get busy enough for a queue to develop for the showers - but it is well managed and there is never a sense of over-crowding.