Koh Phangan or Koh Tao?

Both islands are surrounded by sea that stays close to 29°C, so the difference lies in the kind of beach. Koh Phangan has more of them and bigger ones. Thong Nai Pan and Bottle Beach in the north are soft, deep and calm, and Haad Yao and Haad Salad on the northwest coast are dependable for swimming. The long south coast is a different story, with reef flats that leave the water too shallow to swim at low tide.

Koh Tao has one proper long beach, Sairee, which has gentle water and sunsets. The rest of its coast is small coves like Aow Leuk and Tanote Bay, where the snorkelling starts straight off the sand, but several of them are reached down steep tracks.

Neither island is a reason to travel for food, but Koh Phangan feeds you better. The evening market and Saturday walking street in Thong Sala are good for grazing through southern Thai dishes and roti, and Srithanu adds a cluster of vegan and raw cafés for the yoga crowd.

On Koh Tao, meals mostly happen in Sairee and around Mae Haad pier. Many kitchens cook breakfasts, pizza and burgers for divers, and there is no market to plan an evening around.

Koh Phangan's sights fit easily into one afternoon. They include Wat Phu Khao Noi near Thong Sala, the island's oldest temple, the hillside meditation temple of Wat Khao Tham, a few Chinese shrines in the villages, and the initials King Rama V carved into the riverside rocks at Than Sadet. There are no museums or historic towns beyond that.

Koh Tao has even less: a couple of small temples. Its 1930s past as a place of political exile has left almost nothing a visitor can see. On either island, sightseeing is a short break from the beach rather than part of the plan.

This is where the two islands differ most. Koh Phangan's nights follow the moon: the Full Moon Party fills Sunrise Beach at Haad Rin with tens of thousands of people until after dawn, half-moon and jungle parties are held inland, and the west coast has its own gentler sunset bars and sober ecstatic-dance nights.

Koh Tao has a smaller, friendlier scene at the southern end of Sairee. There are low tables on the sand, fire shows and bar crawls, but no clubs, and the rest of the island is quiet by late evening. It is enough for a few drinks after a day's diving, not a party trip.

Koh Tao is the stronger choice for the outdoors, almost all of it under water. Coral gardens ring the coast, the calm, warm sites suit beginners, and boats run out to Chumphon Pinnacle and to Sail Rock, where barracuda school. On land there are granite boulders for climbing, and short, steep trails lead up to views over the bays.

Koh Phangan's outdoors is more on land. Its forested interior climbs to Khao Ra at about 630m, with waterfalls at Phaeng and Than Sadet, and full-day boats go to the limestone islands of Ang Thong. Sail Rock lies between the two islands, but Koh Tao's dive culture makes it easier to build a trip around diving.

On both islands a day trip means a ferry to a neighbour, and most visitors stay put. Koh Phangan has the easier link: Koh Samui is about half an hour away, with the Big Buddha and the shophouses of Bophut, and Koh Tao itself makes a snorkelling day out. From Koh Tao, Koh Phangan is about an hour away and Samui about two, though Koh Nang Yuan's sandbar is right offshore. Monsoon seas can cut both islands off for a day or two late in the year.

Koh Phangan is the easier island with children, as long as you pick the right coast. Thong Nai Pan, Haad Yao and Haad Salad have gently shelving sand and resorts with pools and family rooms, and the sandbar walk to Koh Ma is a simple outing. Haad Rin and the days around the full moon are best avoided.

Koh Tao works for older children who can swim and want to try a junior dive course in the shallows off Sairee or Shark Bay. With toddlers it is harder, because many bays are reached down rough tracks and serious medical care means a boat to a larger island.

Neither island has public transport. On both you rely on shared pickup taxis, longtail boats or a rented scooter on steep roads where inexperienced riders often crash. Koh Tao is slightly easier to manage on foot, since Sairee and Mae Haad pier are linked by a walk along the beach and a village lane.

Koh Phangan is bigger and more spread out. Thong Sala to Haad Rin takes about twenty minutes by road, and the far eastern bays and Bottle Beach are at the end of winding roads or a boat ride. Reaching Koh Tao in the first place usually takes a long travel day by ferry.

Thong Sala's Saturday walking street is about as far as shopping goes on Koh Phangan, with loose cotton clothes, cheap jewellery and Thai snacks. Srithanu adds yoga clothing, crystals and handmade festival outfits. On Koh Tao the choice shrinks to sarongs, souvenir T-shirts and the dive shops along Sairee and around the pier, so bring anything important with you.

Both are cheap islands where simple bungalows, Thai meals and scooter hire cost little, and on both, imported food and drink cost more because they arrive by boat. The catches differ. On Koh Phangan, rooms in Haad Rin around the full moon cost several times the usual rate and often come with a minimum stay, while a long stay on the west coast can be very cheap. On Koh Tao, dive courses are the bargain, and booking a room through a dive school often lowers the bill further.

There is no theatre or concert hall on either island. Koh Phangan has a little more live music: acoustic and open-mic nights in west-coast bars, reggae jams, and music at Srithanu's ecstatic-dance and sound-healing gatherings. On Koh Tao, live entertainment comes down to fire-dancers on Sairee and the odd acoustic set.

Koh Phangan is the place for a proper wellness stay. Srithanu and its coast are built around yoga shalas, meditation and silent retreats, detox programmes and breathwork, and people stay for a week's course or for months. Wat Khao Tham adds Buddhist retreats, and the northern bays stay quiet day and night.

Koh Tao is restful in a simpler way: a massage under a beach shelter, a yoga class above the bays and days in the water. Pick the eastern bays or Chalok Baan Kao rather than Sairee if you want quiet nights.