The Y seat on both the 34B and 33B is identical. It is the latest fixed back longhaul economy seat that was introduced around 2008. You will find this on long-haul flights.

With regard to the aircraft, The A330-300 will probably be a newer plane than the A340-300 however as both have the latest Y and J products, from a Pax point of view, it shouldnt be too different.

Good luck for getting the J seat on the HKG-JNB leg. If it is an A340, there will be a relativley small business class, but hopefully they will still have space for your mileage upgrade! The J seat will be appreciated on that leg as it is an overnight flight.