Thanks to everyone for the responses. After confirming the baggage fees went ahead and booked BA (operated by Comair).

All 3 of our bags will definitely be packed to 50 lbs, so on Kulula I would be paying 3 x the overweight fee of 200 ZAR. On BA I may not have to pay any overweight fee, but even if the one bag is packed at 70 lbs it will be 1 x 250 ZAR. So the bags alone wipe out 350 of the 750 ZAR price advantage for Kulula.

For 400 ZAR/$50 more on BA we will earn about 3000 miles total (which easily makes up the small price difference) with the upside of potentially using the elite check-in lines.

Also, it looks like I can change the BA flights (keeping the same routing) without a change fee whereas it would be 130 ZAR per person per sector on Kulula. There is a small chance we will move our itinerary up a few days if the award availability opens up on UA/SAA.

One thing I am trying to confirm is if I am actually flying a BA plane JNB-DUR? On a few online booking sites it said my JNB-DUR flight BA 6215 would be operated by Kulula. But BA.com just said operated by Comair.