On this route I think it’s about:
– cost (above all, this can be a pricey route!)
– timing of flights
– availability of seatback AVOD (does SAA offer that?)
– seat pitch
– type of aircraft
– miles earning potential
Also, it would be wise to consider airlines such as Swiss as they could be considerably less expensive, although would necessitate a stopover.
Also, I would suggest checking out Premium Economy and sometimes even Business Class; it can often be less expensive than you imagine to trade up from a peak December economy fare (when leisure demand is high) to a Business Class fare (when business demand is lower).
While it’s best not to be overly wedded to one carrier, you should consider whether your in laws can earn miles in a programme with which they will have a chance of adding to in the future, and redeeming on routes which would also be of interest to them.
The ITA Tool is a good place to start.