The advantage of Khao Lak is that it's on the mainland. So you have easy access to the National Parks, local towns and villages, elephant trekking and other attractions. There is also easy access to sea based activities around The Surin & Similan Islands. I haven't been to Koh Chang, but part of the decision will be the time of year you travel During May-September the Andaman Coast is in it's low season so some restaurants and shops may be closed The Gulf of Thailand is somewhat different. There's also a period during the low season when the marine national parks near Khao Lak are closed. On balance I would say Khao Lak due to the variety of attractions and not being limited to an island, but I am slightly biased.