Hi Chris,
It all depends on what you want to do and how much time you have.
If you’re looking to soak up some sun and go for a swim, Hainan is definitely the better option. It’s also my first choice. Just bear in mind that Riyue Bay is an extremely small town and apart from the beach, there really isn’t much to do.
If you like island hopping and exploring, I’d recommend Zhoushan. These islands also boast a couple of decent beaches, but they don’t quite compare to the beaches in Sanya and Riyue Bay. The main attraction in Zhoushan is Putuoshan.
Sorry for the late reply, hope it still helps.
Enjoy your trip!
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