Taking the coach between the coastal cities is a pleasant and comfortable way to see a lot of countryside (north KZN coast, skirt the Drakkensberg, cross Swaziland, descent into Maputo).
Tickets are easily purchased ahead of time at the Durban terminus. Everybody recommends to use the Mozambique owned service rather than the SA owned one (easy to tell - it has a Portuguese name - something Azul as I recall). That worked for me, comfortable seats, refreshments, air conditioning, helpful staff.
Durban bus station is organised but you won't want to spend much time there as it has the same edgy feel as any major bus terminus in a European or North American city (even on a Sunday morning departure). Border crossings (two because of Swaziland - make sure your visa situation is sorted before travel) are relatively painless. Even the rest stops are interesting.
Maputo bus station is pleasantly chaotic and not too hassled, like that lovely languid seaside city itself (better than the somewhat bureaucratic 1970's southern-Europe style airport). I only got to spend two days in Maputo but it is charming and atmospheric though becoming a playground for South Africans so progressively losing much of it's Luso/Afro distinctiveness. Lots of great food, especially seafood of course. Loads of restaurants, bars and clubs (though Sunday night is quiet) and it will feel quite familiar if you have been to Lisbon, or indeed Macao. It is not compact but laid out with wide boulevards so not very walkable, but taxis are cheap and easy. And you have to love a city that has streets named after Trotsky, Stalin and Kim Il Sung!
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