Koh Lanta or Koh Phangan?

The beaches are equally strong, but they are laid out differently. Koh Lanta's beach life is all on one side: a row of west-facing bays, from the shallow, easy sand of Klong Dao and the long casuarina-lined Phra Ae down to the quieter coves where jungle meets the sand near the national park. That coast is at its best from November to April and largely closes down in the monsoon.

Koh Phangan's bays ring the whole island and each has its own character. Thong Nai Pan and Bottle Beach in the north have deep, calm water, Haad Yao and Haad Salad are the dependable swimming bays, and the long southern shore is better for sunsets because of its reef flats at low tide. With a sheltered side to choose from, Phangan works across more of the year.

Eating is much the same on both: cheap, honest Thai cooking with little fine dining. What differs is the flavour. Koh Lanta's mostly Muslim population puts southern curries, roti and halal cooking on everyday tables, and the stilt restaurants of the Old Town serve crab and fish over the water.

Koh Phangan's best eating is concentrated in Thong Sala, at the evening market and the Saturday walking street. Srithanu adds a scene Lanta lacks, with vegan, raw and smoothie-bowl cafés serving the yoga crowd.

Neither island is worth visiting for its sights, and either can be covered in an afternoon. Lanta's heritage is a community and a trading past: the wooden Chinese shophouses of the Old Town on stilts over the water, and the Urak Lawoi sea-people village at Sang Ka U. Phangan's is religious and royal: its oldest temple near Thong Sala, the meditation temple of Wat Khao Tham, and the kings' initials carved into the rocks at Than Sadet.

Here the islands are far apart. Koh Phangan is a byword for partying. Every month the Full Moon Party fills Sunrise Beach at Haad Rin, half-moon and jungle parties are held inland, and the west coast offers a softer night out with sunset bars and sober ecstatic-dance gatherings. The usual warnings about crowds and drink-spiking apply on the big nights.

Koh Lanta has nothing like it. Going out means cushions on the sand, reggae and a fire show along Long Beach or Klong Dao, and many bars shut outside the dry season. Party-goers staying on Lanta take the boat to Phi Phi.

Koh Lanta has the edge, mostly because of the sea around it. In the dry season, boats run to the limestone islands off the Trang coast and the Emerald Cave on Koh Mook, and divers reach Koh Haa and the pinnacles of Hin Daeng and Hin Muang, where mantas and whale sharks pass. On land there is the national park at the southern tip, the caves of the forested interior and the mangroves of the east coast to explore by kayak.

Koh Phangan has more to offer on land. Steep jungle trails climb to Khao Ra at about 630m and pass waterfalls such as Phaeng and Than Sadet. Offshore, Sail Rock and the lagoons of Ang Thong are fine outings, but they are less remarkable than what lies off Lanta.

Neither island offers much in the way of day trips, and on both they mean a boat. From Koh Phangan, Koh Samui is only half an hour away and gives a genuine change of scene, with the Big Buddha and Fisherman's Village in Bophut, while Koh Tao suits a snorkelling day.

From Koh Lanta, crowded Phi Phi and the gentler fishing village on Koh Mook are the main outings. Krabi Town and Ao Nang put the mainland within reach, but the choice shrinks sharply in the monsoon.

Koh Lanta is the more straightforward choice with children. The whole island is calm, Klong Dao has soft sand and shallow water in the dry season, and days fall easily into beach, pool and early dinner. The monsoon months are the exception, when the seas become unsafe for swimming.

Koh Phangan can work well, but only in the right place: Thong Nai Pan or Haad Yao and Haad Salad, well away from Haad Rin and the full-moon dates. The steep scooter roads add to the care needed. Neither island has an aquarium, a zoo or a theme park, and on Lanta serious medical care means a trip to Krabi.

Both islands are hard work without your own wheels, but the problems differ. Koh Lanta is a long strip with one road down the west coast and no buses, so you rely on sidecar motorbike taxis or a hired scooter, and the island takes two to three hours to reach from Krabi airport.

Koh Phangan is smaller and has shared songthaews and app taxis. Its roads are steeper, though, with some gravel stretches, and Bottle Beach is easiest by longtail. Thong Sala to Haad Rin is only about twenty minutes.

A sarong, a gift or some beachwear is about as far as shopping goes on these islands, so the difference is in what kind of stall you find. On Koh Lanta it means souvenir and beachwear stalls along the main road, the small night markets around Saladan, and a few batik and craft shops in the wooden houses of the Old Town.

Koh Phangan's shopping follows the island's two crowds. Thong Sala's Saturday walking street sells loose cotton clothes and cheap jewellery, Haad Rin sells neon party wear, and Srithanu sells yoga clothing, crystals and handmade festival outfits. For malls or design shops, wait for Bangkok.

The two islands cost about the same, and both are cheap by Western standards for bungalows, local food and massages. On Koh Lanta, the quieter south and east coast cost less than the northern beaches, Christmas and New Year are the expensive weeks, and transfer fares add up without public transport.

On Koh Phangan, a long stay on the west coast can be very cheap. The costly parts are the boutique bays of Thong Nai Pan and Bottle Beach and, above all, Haad Rin around the full moon, when rooms are booked out and cost several times the usual rate.

There is little on either island, but Phangan offers slightly more. Its west coast has open-mic and jam nights, acoustic sets in beach bars, and the music that goes with its ecstatic-dance and sound-healing gatherings. On Lanta, live entertainment means fire twirling on the sand, an occasional reggae band in high season and Muay Thai bouts at small local rings.

Both islands are made for slowing down, but Koh Phangan treats wellness as a reason to travel. Srithanu and its coast are full of yoga shalas, silent and meditation retreats, detox and breathwork programmes, and people come for a week's course or stay for months. Wat Khao Tham has long offered Buddhist retreats.

Koh Lanta's restfulness comes from its pace rather than from a scene: open-air massage pavilions on almost every beach, resort spas, and yoga studios around Long Beach. The coves around Kantiang Bay are the calmest place to stay.