i have dived all four places.
Of the four, I think Aliwal and Sodwana are more interesting. Aliwal is guaranteed shark diving. Also some wrecks in good condition. Sodwana has excellent reefs. If you go to Sodwana try to dive with “Da Blue Juice”. A small Op with personal service. Coral Divers is the biggest Op there, if you need to hire gear etc.
@tridacna may have other suggestions if you are interested in the Cape.
Mozambique will need more than 6 days because of travel issues. Travel within Mozambique, while safe, can be unreliable. Our return flight from Pemba to Johannesburg was simply cancelled while waiting to check-in at the airport and I had to do all the rebooking via Maputo myself, including having to trek to the airline office in the city, to get my sons back to school in the UK. Fortunately I had allowed 2 days layover in Johannesburg, so they didn’t lose their Jhb-Lon flights, which would have been more expensive. Personally, I wouldn’t try Mozambique unless I had more time.
PS: it will be winter then, but I was fine with a 5mm wetsuit.