I've been to Amanoi and it's amazing. We're returning late this year. Here's a thread about it; search on Amanoi in the title and you'll find more.

https://www.flyertalk.com/forum/luxu...ip-report.html

Here's a thread on Six Senses Ninh Van Bay. I haven't stayed there and can't compare the properties. I am reminded that I decided NOT to stay there based on the thread. It looks very rustic compared to the exceptional beauty of Amanoi.

Trip report: Six Senses Ninh Van Bay, Vietnam