Koh Samui or Phu Quoc?

Both islands are built around their beaches, and the sand is equally good on each, so the real difference is layout and season. On Samui, the busy east coast beaches of Chaweng and Lamai and the sheltered bay at Choeng Mon are best from January to September. The calmer north coast turns shallow at low tide, and boulder coves like Silver Beach are the prettiest places to swim.

Phu Quoc's Long Beach is a single 20km sunset strip down the west coast, calm only from November to April. The finest white sand is out at Sao and Khem in the southeast, and some stretches away from the resorts are scruffier than the photos suggest.

The two are about level, and both are best eaten cheaply and locally. What changes is the cooking. Samui's is southern Thai, with hot curries, coconut soups and the grill stalls of the Fisherman's Village walking street, plus a layer of polished sea-view restaurants in Bophut and the hillside resorts.

Phu Quoc is all about seafood picked live from the tank, raw herring salad rolled in rice paper, and stilted restaurants over the water at Ham Ninh. It also has food to visit as well as eat, since you can tour the fish sauce houses and pepper farms the island is known for.

Neither island is a place for heritage, and either can be covered in half a day. Samui's sights are visual and religious: the gilded Big Buddha on its islet, the many-armed Guanyin at Wat Plai Laem and the mummified monk at Wat Khunaram.

Phu Quoc has fewer temples but more history. The old prison near An Thoi tells the story of the French and American wars, and it is the most sobering site on either island. Its replica European streets in the resort districts are fun to walk but nothing more.

Samui is far livelier after dark. Chaweng's beach clubs, fire shows and clubs in the lanes behind the sand run late through the high season, Lamai has a rougher bar strip of its own, and the full moon parties on Koh Phangan are a short boat ride away.

Phu Quoc's evenings are sundowners on Long Beach, a few backpacker bars in Duong Dong and crowds strolling the lit-up entertainment districts, with a casino in the north. It suits people who want dinner and an early night rather than a party.

Samui has the better outdoors, almost all of it at sea. A boat day to Ang Thong's forty-odd limestone islands, with kayaking, snorkelling and the climb to the viewpoint over the emerald lake, beats anything near Phu Quoc. Divers have Sail Rock and the waters around Koh Tao.

Phu Quoc does have the larger protected area, with forested national park hills covering much of the north. But its southern reefs are pleasant rather than memorable, and its best view is from the Hon Thom cable car rather than from a trail.

Neither island is a good base for trips away, but Samui has more choice. Koh Phangan's quieter beaches and Koh Tao with the sandbar islets of Nang Yuan each make a proper day out by boat.

From Phu Quoc, the island-hopping boats around the An Thoi archipelago are really beach and snorkelling stops. The only mainland outing is the ferry to Ha Tien, a quiet border town with a temple hill.

Phu Quoc is the easier family island because it has things to do beyond the beach: a safari park where buses drive through the enclosures, a theme park with rides and an aquarium, and a water park reached by cable car. On the downside, these are spread across the island, so each outing means a taxi.

Samui is fine for a resort stay in calm Choeng Mon, Bophut or Maenam, with kids' clubs and famously warm staff. But beyond small animal parks and boat trips for older children, the offer is thin, and the busy ring road is no place for family scooter rides.

Phu Quoc is the simpler island to get around without your own wheels. Clean electric buses run along the main spine from the airport through Long Beach and Duong Dong to the north, and metered taxis and ride-hailing cover the rest. Distances are longer, though, and most outings are a half-hour ride.

Samui is more compact, and its single ring road puts most beaches within a quarter of an hour of each other. But the songthaews keep no timetable and taxi fares have to be haggled, so most visitors end up hiring a car or riding a scooter.

Samui offers a little more to browse. The Friday walking street in Fisherman's Village has crafts and coconut-wood homeware, tailors make suits to order, and Chaweng has a large modern mall.

On Phu Quoc, shopping comes down to cultured pearls, black pepper and dried seafood from the Duong Dong market. Check before packing fish sauce, which airlines may refuse.

Phu Quoc is the cheaper of the two. Guesthouses near Duong Dong, market seafood and metered taxis cost little, though the theme parks, the cable car and the big resorts add up quickly.

Samui is one of Thailand's dearest places to stay, with expensive taxis, resort-priced drinks on the main strips and limited flights into its small airport. To keep costs down, stay on the north or west coast and eat at the night markets.

Neither island has theatre or serious music, so this is a choice between kinds of show. Samui's is rougher and closer up: Muay Thai fight nights around Chaweng and Lamai, cabaret and fire dancers on the sand. Phu Quoc puts on large, polished sea-front spectacles with fireworks, water effects and drones in its resort districts.

Samui is the place for a restorative trip. Lamai and its hills have long been known for detox and fasting programmes, there are yoga and meditation retreats across the island, and the slow north and west coasts around Maenam and Lipa Noi make it easy to switch off. Just stay away from Chaweng.

Phu Quoc is relaxing in a simpler way. There is cheap Vietnamese massage, herbal baths in resort spas, and quiet bases at Ong Lang or near Khem Beach, but no real retreat scene.