I've been to both places though not for many years. I found Langkawi more interesting with more diversity in what to see and do. Phu Quoc (at the time that we went) was mostly hotels on the beach. We took a tour in an ancient American military jeep and went to a place that made fish sauce. Otherwise, we didn't find activities off our resort. I'm sure a lot has changed since then, though my perception is that the island is being heavily developed for tourism.