I‘m presently in the final stage of application (background check) for Hainan. I applied as a DEC on A330. One of the main reasons for me was that you will also operate on international/intercontinental (expect 1-2 longhaul a month) routes compared to an A320 (or 737) contract, so not spending all your ON time only inside China.
Rumours about increased contract terms for HNA in 2019.
Anyway I think the biggest money will always be by at A320/737 operators based in a sh.... second/3rd tier city.
I don‘t have any insider information on Xiamen.
Good luck for the application!