Six Senses Con Dao: Amazing beach, resort itself is above average.

Amanoi: Average (on a good day) beach, resort itself is probably the best in Vietnam.

Also keep in mind that for less than the cost of a pool villa at Amanoi, you can book a two bedroom residence villa at CD, which will have much more space, a huge pool, and will be right on the beach.

I think it's a matter of prioritizing what is most important to you and making a decision based on that. Or go to both (they make a great combo).