OP here. Old thread, but I'm back from my honeymoon and wanted to thank everyone for their advice and give a quick report.
Against many recommendations, I decided to go with FS Samui. It was by far the easiest travel option, with short flights that aligned perfectly with our existing travel plans. Samui's size also fit well with the # of days we had available; other suggestions liked Bali and Vietnam each seemed worthy of their own trip.
I booked direct with FS since they had a lower rate than Amex and Virtuoso, and promised many of the same benefits (room upgrade, free breakfast, etc).
I was quite nervous heading in, but our stay was absolutely amazing. I'll try and avoid re-hashing the positives other reviewers have commented on (like the stunning location) and only comment on negatives I was afraid of -- most of which did not materialize.
Pros: On arrival, we were greeted by the resort manager. He was very welcoming, noted it was our honeymoon, and said he'd upgraded us to one of his favorite villas. I was skeptical, but our unit was truly outstanding and had completely unobstructed views of the ocean. The welcome rose petals and bottle of champagne were also a nice touch. All units have private plunge pools. Ours was the perfect temperature the entire day, and we spent a LOT of time in it. I was very glad we had our own pool, as opposed to having to travel to a shared one. Service was excellent throughout the stay. We were only asked our villa number once, despite many trips around the resort. Villa itself was beautiful and comfortable.
Mehs: Food was solid, but not amazing. I expected that, though, for a resort. There wasn't loads to do, but we were honeymooning, so we didn't care. I did read one review complaining about housekeeping showing up whilst they were in unit. Thishappened once to us as well, but in their defense, we had basically been in our unit the entire day.
We said no, come back later, at which point they apologized and left immediately (and indeed returned while we were at dinner). Didn't really negatively impact our experience in any way.
In short -- incredible stay.
Again, all the advice in this thread was much appreciated. Cheers.