W Koh Samui beach is not that great. Your best bet on Koh is the Conrad or Four Seasons. However if you must stay at a Starwood property then Vana is your best bet.

Same thing on Phuket. There are so many better properties when you look outside of Starwood.

Lastly I would consider StR Bora Bora.

Best advice is it is your honeymoon and do not limit your thinking to Starwood. Just pay for the room you want, so you don't need to worry about the upgrade game.