Koh Phangan or Phuket?

Both are first-rate beach islands, and the real difference is in the sea and its seasons. Koh Phangan's best swimming is in the north and northwest, at Thong Nai Pan, Bottle Beach, Haad Yao and Haad Salad. Its southern shore turns to shallow reef flats at low tide, and the east coast gets rough from about October to December.

Phuket has a longer chain of sandy bays along its Andaman coast, running from crowded Patong to small coves such as Kata Noi and Nai Harn and the long, quiet northern beaches. Its rough season is the opposite one: from May to October surf and rip currents bring out the red flags.

Phuket has a cuisine of its own, and that is the clearest gap. Its Hokkien and Peranakan dishes are eaten in Phuket Town's shophouses and markets from breakfast onwards, and together with the seafood at Rawai and a small fine-dining tier they make eating out a reason to visit.

Koh Phangan eats well but simply. Thong Sala's night market and Saturday walking street serve southern Thai staples, and Srithanu has vegan and raw cafés for the yoga crowd. There is almost no fine dining, and menus in the resort bays are shaped for visitors.

Neither island is a culture trip, but Phuket at least has a town worth exploring. The Sino-Portuguese shophouses of its old quarter, Wat Chalong, the Big Buddha and the Thalang museum's story of the 1785 Burmese siege fill a day without strain. Koh Phangan has a few temples, such as Wat Phu Khao Noi and the hillside Wat Khao Tham, and the royal initials carved into the rocks at Than Sadet, and an afternoon covers them all.

Koh Phangan has the bigger name for partying, because its nights are events rather than a street. The Full Moon Party on Haad Rin's Sunrise Beach and the half-moon and jungle parties inland draw young backpackers from around the world. Srithanu offers a gentler night out, with sunset bars and sober ecstatic dance.

Phuket's nightlife is steadier and less of a spectacle. Bangla Road in Patong fills with neon bars, go-go bars and big clubs every night of the high season, and there are beach clubs on the west coast and shophouse cocktail bars in Phuket Town. On both islands the north goes quiet after dinner.

Phuket's outdoors are more striking, and most of them are at sea. You can kayak through the caves and lagoons of Phang Nga Bay's limestone towers, and the Racha and Similan islands offer diving with reef sharks and rays. On land, Khao Phra Thaeo protects a patch of rainforest with gibbons.

Koh Phangan's hilly interior is emptier, with trails up Khao Ra at about 630m and waterfalls at Phaeng and Than Sadet. Offshore it has Sail Rock, a fine dive site, and boat days to Ang Thong National Marine Park, though overall there is less to do.

Phuket has more choice beyond its shores, partly because a bridge joins it to the mainland. Boats go to Phi Phi and to the stilt village of Koh Panyee in Phang Nga Bay, and by road you can reach Krabi and Ao Nang or the old tin town of Takua Pa.

From Koh Phangan, every outing is a ferry. Koh Samui has the Big Buddha and Fisherman's Village in Bophut, and Koh Tao is good for snorkelling at Koh Nang Yuan. Beyond those two islands, little is within easy reach.

Phuket is far easier with children. The big resorts at Kata, Karon and the northern beaches have kids' clubs and lagoon pools, and the water parks, the Cape Panwa aquarium and island boat trips help with restless days. Avoid the west-coast surf from May to October.

Koh Phangan suits families who want calm sand and little else. Thong Nai Pan, Haad Yao and Haad Salad have gentle water and family rooms, and the sandbar walk to Koh Ma is a treat. There is nothing built for children, the roads are steep, and Haad Rin around the full moon is best avoided.

Phuket has a little more structure for getting about, though on both islands most visitors end up on a scooter or in a taxi. Koh Phangan has no proper public transport. Shared songthaews wait at the Thong Sala pier and in the villages, the hill roads are steep and partly gravel, and Bottle Beach is easiest to reach by longtail boat. The island is small, though, and Thong Sala to Haad Rin takes about twenty minutes.

On Phuket, songthaews run from Phuket Town out to the main beaches, an airport bus covers the west coast, and taxis and ride-hailing are easy to find. They are costly, and the coastal roads clog up in high season, so most people stay at one beach and book rides for outings.

Of the two, only Phuket can fill an afternoon of shopping. It has malls in Patong and near Phuket Town, tailors who make clothes in a few days, the Sunday walking street, old-town boutiques selling batik and Peranakan-style goods, and pearls from nearby farms. Koh Phangan has little beyond the Saturday walking street in Thong Sala, party wear in Haad Rin and yoga clothes and crystals in Srithanu.

Koh Phangan is a little cheaper day to day, especially on a long stay on the west coast with a simple bungalow, Thai food and a hired scooter. The exceptions are Haad Rin around the full moon, when rooms cost several times the usual rate, and boat transfers, which add up.

Phuket is one of Thailand's more expensive places. Taxis and tuk-tuks charge fixed fares, and resorts charge high-season rates from December to March. Staying in Phuket Town or at Kata and Karon and eating locally keeps the cost reasonable.

Neither island is a place for theatre or concerts, but Phuket has more of an evening out. There are cabaret revues in Patong, a large cultural theme-park show, Thai boxing nights and cover bands in Kata and Phuket Town. Koh Phangan's live music is smaller and looser: acoustic and open-mic sets around Srithanu, reggae jams, and fire-dancers on the sand at Haad Rin.

Both islands are excellent for slowing down, but in different ways. Koh Phangan is about inner practice. Srithanu has yoga shalas, silent retreats, breathwork and sound healing, Wat Khao Tham offers meditation, and many people stay for weeks or months.

Phuket leans towards pampering and fitness, with resort spas in open-air pavilions, detox programmes and Muay Thai camps around Chalong. For peace and quiet, stay at Mai Khao, Bang Tao or Nai Harn rather than Patong.