Having lived in Hawaii and holidayed in Sanya around 2.5 years ago, I would not recommend Sanya over Hawaii.

The climate is not as warm, as it is further away from the equator. The beaches are crowded, and can be dirty (construction waste and trash from other visitors). The hotels will be more expensive in peak season, and if you go during or close to Chinese New Year it will be rammed with Northern Chinese trying to get away from the cold for New Years. Expect to pay £500+ for a room in a 5 star property then.

The reason it is popular with the Chinese is the lack of a need for a visa.