Hi Mystic
Hard one really, as they are two excellent resorts but two very different locations. My personal selection between them would be the Merlin Beach resort but as I said that is my personal opinion. Were you offered any other choices? And what else do you want from your stay, such as location and facilities etc? that could help in giving you advice on which resort might better suit you.
The location of Movenpick is not really my favourite spot in Karon so that is my main reason for not picking that resort but if you want to be within walking distance of restaurants and shops such as chemists, convenience and a long stretch of beach then movenpick is the one. If you dont mind being away from the hustle and bustle but still close enough to get a shuttle or Tuk Tuk into town then Merlin beach would be the best option. I hope this can help a little. If there is something specific that you are looking for in the resort that you want to stay at let us know and someone should be able to help you.
Cheers.
Darrin.