Does anyone have recent experience with the differences in the A340-300 and A340-600 fleet? Booking an award in J and the equipment LHR-JNB is a A340-300 both ways (there is also a A330 flight on the same route that could work).

My question, on the SAA website, it lists the J seats as "Premium Class Flat Bed Seat" but on the -300, it has in fine print "Seat is fully flat with backrest almost fully flat, with a slight incline". What are the details on this? Is the -300 to be avoided for this reason on a 10hr flight in December?

Thank you!