Samui to me is packed with places trying to separate you from your baht, it is a high destination area and it is very touristy. I enjoyed my time there but there are much better places. Krabi of course is still touristy and everyone is wanting to separate you from your baht but Krabi to me, is more romantic or more enjoyable for a honeymoon. There are some great places down south, Khao Lak I went with a lady and had a perfect few days of seclusion.

There are literally hundreds of places down south and in Thailand for that matter that are more romantic then Samui or Krabi.

Would I go back to Samui, sure if they give me a free room and airfare. Would I go back to Krabi... probably some day, been there more then a few times as you gotta fly into Krabi, Phuket or Kantang if you want to get down south by air.

Expand your search maybe. There was a semi decent book by Lonely Planet, Thailand's Islands and Beaches that might help you more to know about which destination to pick.