Koh Phi Phi or Koh Tao?

Both islands have clear water at about the same warmth, but Phi Phi's beaches are the more spectacular. Long Beach looks across to the cliffs of Phi Phi Leh, Laem Tong in the north has soft sand and calm, quiet water, and boat trips add Bamboo Island and the coves around Maya Bay. The one weak spot is Loh Dalum by the village, which turns very shallow at low tide.

Koh Tao has one long strand, Sairee, with about 2km of soft sand, gentle water and sunsets over the Gulf. Elsewhere the coast breaks into small coves such as Tanote Bay, Aow Leuk and Freedom Beach. They are good for snorkelling straight from the shore, but several are reached down steep tracks.

Neither island is a reason to travel for food, and both cook mostly for visitors with ingredients brought in by boat. Phi Phi's best moments are seafood laid on ice and grilled on the beach in the evening, plus banana roti from carts in Tonsai village. Koh Tao's eating in Sairee and around Mae Haad pier is built around divers' routines, with breakfast cafés, fruit shakes and a few night stalls selling grilled skewers. A meal on Koh Tao is usually the cheaper of the two.

There is next to nothing to see on either island, and on both the trip is about the sea. Phi Phi offers a small tsunami memorial and a view from a passing boat of Viking Cave, where swiftlet nests are gathered. Koh Tao has a couple of small temples and a history as a place of political exile in the 1930s, though almost nothing of that past is left to visit.

Phi Phi is the bigger party island. Every night the bars along Loh Dalum lay out mats, put on fire shows, play loud dance music and serve buckets of drinks until the early hours, while the village fills its reggae bars and stages amateur Muay Thai bouts. It is young and fairly uniform, but it is the main event of an evening here.

Koh Tao's nightlife is a smaller version of the same idea, centred on the southern end of Sairee Beach with fire-dancers, bar crawls and themed beach parties. There are no proper clubs, the rest of the island goes quiet early, and the scene thins out in the rainy months at the end of the year.

Above the water, Koh Phi Phi wins clearly. Sheer limestone walls rise straight out of the sea, the climb from the village to the viewpoint over the twin bays pays off at sunset, and kayaks slip under the cliffs of Phi Phi Leh into the enclosed Pileh Lagoon. Climbers have bolted limestone cliffs near Tonsai, and nothing on Koh Tao looks like this.

Underwater the balance tips the other way. Phi Phi has soft-coral walls at Bida Nok and Bida Nai and blacktip reef sharks at Shark Point, but heavy boat traffic has worn its reefs. Koh Tao is ringed by shallow coral gardens, and its granite pinnacles such as Sail Rock draw schools of barracuda and the odd passing whale shark. On land Koh Tao offers wooded hills and granite boulders for bouldering, which makes for pleasant walks but not a view anyone travels for.

From either island, any real day away means a ferry, and most people stay put. Phi Phi's crossings lead to Koh Lanta's stilt-house old town, the cliffs of Railay and the Sino-Portuguese shophouses of Phuket Town, which makes a long day. Koh Tao's options are Koh Phangan and Koh Samui, with towns, temples and markets, and closer in the sandbar islets of Koh Nang Yuan. Phi Phi's boat tours around nearby uninhabited bays give its outings a bit more variety.

Phi Phi is slightly easier with children, provided you stay away from the village. The resorts at Laem Tong have pools and calm, shallow water, and with no cars on the island there is no traffic to worry about. Tonsai stays noisy until late, however, and the ferry can be rough outside the dry season.

Koh Tao suits older children who already swim, and many dive schools run courses for kids. With toddlers it is harder going because of steep tracks to the bays and roads busy with novice scooter riders. Neither island has attractions made for children, and on both serious medical care means a boat ride.

Phi Phi has no roads for cars, which makes the village area easy. Tonsai and Loh Dalum are a few minutes apart on foot, and longtail boats cover Long Beach, Laem Tong and the outer bays. If you stay at a northern resort, book the boat transfer in advance.

Koh Tao is more awkward. The walk from Sairee to Mae Haad pier is easy, but beyond it you rely on shared pickup taxis or scooters on steep, rutted roads where accidents are common. Reaching the island also takes longer, usually a ferry from Chumphon or Koh Samui, against Phi Phi's shorter crossings from Phuket and Krabi.

Neither island is a place to shop, so bring what you need. On both you will find beachwear, sarongs, T-shirts and snorkel gear. Phi Phi's village lanes add a few tailor and jewellery stalls, and Koh Tao's shops lean heavily towards diving equipment.

The gap is widest in where you sleep and how you spend the day. A dry-season room on Koh Phi Phi costs far more than one on the mainland, and the extras pile up: drinks on the beach, longtail rides between bays, boat tours and the fee to enter the island and marine park. On Koh Tao the day's main activity, diving, is the bargain, with courses among the cheapest anywhere for the quality of teaching.

At both ends of the market the islands are closer. Everything on both arrives by boat, so imported food and drinks cost more than on the mainland, and Koh Tao's hillside resorts with pools charge international prices much as Phi Phi's northern beach resorts do. Backpackers get by on either, in dormitories and at roti carts on Phi Phi or in simple Thai kitchens on Koh Tao.

Fire-dancing on the sand after dark is as close as either island comes to a show. Koh Phi Phi's village bars add acoustic and cover acts, and Koh Tao manages the odd acoustic set in a bar. Neither has a theatre, a concert venue or traditional dance, so anyone who wants a performance should look for it on the mainland.

The two are broadly level, and on both the calm depends on where you sleep. Phi Phi is quiet at Laem Tong and the isolated eastern bays, while Koh Tao is quiet in the eastern bays and around Chalok Baan Kao, and both are loud near their party beaches. The difference is in what is on offer. Phi Phi's northern resorts have proper spas set among the trees, along with massage on mats along the sand. Koh Tao leans towards yoga classes above the bays and small retreats inland.