Koh Lanta or Koh Tao?

Both islands have warm, clear sea, but Koh Lanta has more beach. Its whole west coast is a run of bays facing the sunset. In the north are the easy, shallow sands of Klong Dao and the long, casuarina-lined Phra Ae. Further south are the quieter Klong Nin and Kantiang Bay, where the jungle reaches the sand. The catch is the season: from May to October, waves and currents shut much of it down.

Koh Tao has one long beach, Sairee, and otherwise small coves such as Tanote Bay and Freedom Beach, several of them down steep tracks. In return you can snorkel straight off the sand, walk the sandbar at Koh Nang Yuan, and find calm water through the middle of the year.

Koh Lanta has more food that belongs to the island itself. Most of the people who live there are Muslim, so a meal from a local kitchen is often a southern-style curry with flaky roti, halal as a matter of course. The Old Town on the east coast adds crab and fish eaten on stilts over the water, and in the evenings there are seafood grills on the beach and night markets around Saladan. The range is still narrow, with little fine dining.

Koh Tao feeds a dive crowd. In Sairee and around the pier at Mae Haad, menus lean towards breakfast cafés, pizza and burgers, with simple Thai kitchens and a few stalls of grilled skewers and pancakes. It is pleasant refuelling, but nothing worth planning a meal around.

Heritage is thin on both, but Koh Lanta at least fills a half day. There are the wooden Chinese shophouses of the Old Town on stilts over the water, left from its trading days between Penang and Phuket, and the Urak Lawoi sea-people village at Sang Ka U. Koh Tao has a couple of small temples, and its past as a place of political exile left almost nothing to see.

Koh Tao is the livelier island after dark. Young divers and backpackers fill the bars on the sand at the southern end of Sairee Beach, with fire-dancers, bar crawls and themed beach parties into the early hours, though there are no real clubs.

Koh Lanta's evenings are slow on purpose: cushions on the sand along Long Beach and Klong Dao, reggae, a fire show and an early night. Anyone staying on Lanta who wants a bigger night takes the boat to Koh Phi Phi.

Both islands are at their best in the sea, but in different ways. Koh Lanta is a base for boat days, with snorkelling and kayaking among the limestone islands of the Trang coast, including the Emerald Cave on Koh Mook. Divers head out to Hin Daeng and Hin Muang, where mantas pass. On land it adds a national park, caves and a mangrove coast to the east, though the boats mostly stop from May.

On Koh Tao the coral starts right at the shore, and dive boats run out to Chumphon Pinnacle and Sail Rock, with the clearest water from about March to September. Ashore, its granite boulders are good for bouldering and rock climbing, and short, steep trails climb to viewpoints.

Neither island makes a good base for touring, and on both most outings go by boat. From Koh Lanta the choice is mostly island-hopping: the famous bays of Koh Phi Phi and the fishing village on Koh Mook, with Krabi Town and Ao Nang about two hours away on the mainland.

Koh Tao's better days out are to Koh Phangan and Koh Samui, larger islands with towns, temples and markets, while Koh Nang Yuan is a short beach outing. On both islands rough monsoon seas cut the choice, on Lanta from May and on Tao late in the year.

Koh Lanta is much the easier island with young children. It is calm, the northern beaches have soft sand and shallow water, and pool bungalows are common. There is a small hospital near the Old Town and there are clinics in Saladan, though serious care still means a trip to Krabi.

Koh Tao works best with older children who can swim, and many dive schools teach children's courses. Toddlers are harder work: beaches down steep tracks, roads busy with inexperienced scooter riders, and a boat to another island for serious medical care.

Koh Lanta is long and Koh Tao is small, and that shapes every journey on each. On Lanta a single road runs down the west coast, growing hilly and narrow towards the national park, and little is walkable beyond your own beach. To move between bays you take a sidecar taxi or hire a scooter or car. Arriving takes two to three hours by road and car ferry from Krabi airport.

On Koh Tao you can walk from Sairee to the pier at Mae Haad along the sand. Further afield there are shared pickup taxis and longtail boats, since neither island has buses. The steep, rutted roads to the east coast are no place to learn to ride a scooter, and getting there means a long day on the ferry from Chumphon or Koh Samui.

Shopping is no reason to visit either island. Koh Lanta has a little more variety, with craft and batik shops in the Old Town's wooden houses and night markets around Saladan. Koh Tao's shops sell mostly beachwear, T-shirts and dive gear, along Sairee and near the pier.

Both islands are cheap by world standards, and simple bungalows, Thai meals and massages cost little on either. On Koh Lanta, the quieter south and east coast are cheaper than the northern beaches, though taxi and transfer fares add up.

On Koh Tao, the dive courses are the bargain, and booking a room through a dive school often costs less. Groceries and imported food and drink cost more there, because everything comes in by boat.

Neither island has a performing arts scene. On both, the entertainment is fire-twirling on the sand and the odd acoustic set in a bar. Koh Lanta also has Muay Thai bouts at small local rings.

Koh Lanta is the island to slow down on. Open-air massage pavilions sit on almost every beach and yoga studios and retreats gather around Long Beach and the southern bays. Kantiang Bay is about as calm as a stay gets, with little traffic and early nights.

Koh Tao has massage shelters, yoga classes above the bays and a few hillside spas. Sairee is loud at night, though, so a restful stay there means the eastern bays or Chalok Baan Kao.