Jeju or sanya (hainan)

For a nice vacation to nearby countries. I have choices between hainan or jeju. I want sun, sea, nice hotels. Has anyone been to both hainan and jeju which one was better? Or what were the main differences? The hotels in both island are generally good? Ive been to phuket the hotel was hit or miss (mine wasn't even really that cheap it was 200euro per night ). Is the same case ? How clear is the sea?