The relationship between Kulula and BA domestically is an interesting one, since they are both operated by Comair. I have previously booked Kulula flights and flown on BA metal (not sure if planned or due to ops rectification measures). Naturally I was happiest about the meal and wine on offer on the BA plane.

I would say that as far as low cost carriers go, Kulula probably has the best service and overall flight experience of all in South Africa. I also feel more satisfied flying in Kulula's New Generation 737's than I do in BA/comair's older 737s. Inside the BA planes don't seem old at all though. BA/Comair lives up to its licensor's standards in terms of service, whereas you'll have to purchase snacks from Kulula (the flight isn't very long though).

One consideration may be the fact that you would pay both the lower Kulula baggage fee and the higher BA fee if the tickets were separate. Another is your connection time and whether you will have ample time to collect and recheck. Of course if you meant that you would spend some time in Durban rather than simply connecting then these points are irrelevant.

IMO you should go with the cheapest option. You won't be missing much on the short flight between JNB and DUR. Hopefully other posters can be more specific about these flights, I usually only fly CPT-JNB.

Enjoy the time in SA!