Vietnam or Thailand?
Phu Quoc in Vietnam or Koh Samui in Thailand? For a month or a few.
Phu Quoc is a tropical haven in the Gulf of Thailand. It is 31 miles from Vietnamese mainland and less than an hour flight from Ho Chi Minh City. Beautiful beaches, pristine environment and warm people make this place popular. You can snorkel and scuba dive here. Most parts of Phu Quoc is dedicated to National Park so there are lots of places to visit. Peak season starts in November and ends in March.
Koh Samui tourism exists long before other islands in Thailand did. You can enjoy the beach resorts, relaxing spas, intentional cuisine and beach parties. If you are on a tight budget, you can explore the streets and find several street-side food stalls. Backpackers normally flock them. Immerse yourself in authentic Samui cuisine and head to the western or southern part of the island.
Phu Quoc climate is milder. It has fewer cars, and mostly bikes, but less civilization, fewer parties, no movie theaters, no supermarkets and worse value for money. You can find some stuff cheaper (like some fruits, definitely meat).
I would say, if you are okay without big supermarkets, cinemas, big parties, a lot of people, then go for Phu Quoc. Especially if you were to Thailand, Samui and Phangan. If you did not try it yet, think of Thailand, which offers a better variety.