I've wanted to go to Koh Lipe for a while actually, but it isn't so easy to get there. I think you have to fly to Hat Yai, then take a mini bus or taxi to the pier which takes 1.5-2 hours, and then a speed boat which is another 1.5-2 hours. Not sure if there is any ferry service. Definitely is on the cards at some point!
Koh Samui is one of my options, but I'll keep doing some research as to the hotels, transport etc. At least there is an airport on the island which makes for easy travelling.