Carribean is a long way to travel just for 3 nights.

I would prefer SA since it is an overnight flight there and back so it maximises you days there (i.e. you get 4/5 days resulting from the overnight flights). You waste 1 day getting out to the Carribean and half a day on the return.

Domincan Republic is full of all-inclusives. I think Barbados has a bit more style.

Remember to try the rum-cake whichever island you choose.