I was at Rayavadee in May (2nd trip there), and the service and property is amazing. Even with rain/monsoon (which could happen on Samui or at Krabi) the scenary and landscape at Rayavadee is spectacular.

Samui provides a greater number of things to do with some bigger towns, so if you are only going to 1 place in Thailand and that is important... The restaurants are expensive for dinner at Rayavadee, but also very good. Mrs. TioJelly and I did a couple dinners onsite ($150+) and a couple dinners on the beach at other places (<$10).

Bring some ratty clothes if you go to Rayavadee so you can climb to the princess lagoon -- you will get filthy if it has rained -- it's also a bit dangerous, but fun. If they are not really busy when you get there and they know it's your honeymoon they could upgrade you to a room with a private "plunge" pool which is really nice.

Last note -- Be careful not to leave any food out or the monkeys will leave a mess around your place.