Koh Samui or Koh Tao?

Samui has the bigger, broader beaches and more choice between them. Chaweng's long curve of fine white sand and Lamai next to it give plenty of room. Choeng Mon is a sheltered bay for slow swimming, Bophut and Maenam on the north coast are calmer, and boulder coves such as Silver Beach are lovely for a dip. In the monsoon months the north and west shores stay usable while the east coast churns.

Koh Tao's beaches are smaller and more scattered. Sairee is the one long strand, about 2km of soft sand with shallow water and sunsets. The rest are coves such as Tanote Bay and Freedom Beach, some reached down steep tracks. What they give back is snorkelling straight off the sand.

Samui eats better. Its southern Thai cooking of fiery curries and coconut soups is at its best in roadside kitchens and at the night markets of Lamai, Maenam and Fisherman's Village, and resorts and the Bophut waterfront add a layer of polished sea-view dinners.

Koh Tao's meals are cheerful refuelling for divers in Sairee and Mae Haad: Thai basics alongside pizza, burgers and breakfast cafés. There are only a few night stalls and no market worth planning an evening around.

Neither island is a heritage trip, but Samui at least fills a half-day. You can see the gilded Big Buddha on its causeway islet, the many-armed Guanyin at Wat Plai Laem, the mummified monk at Wat Khunaram and the old Chinese shophouses of Fisherman's Village.

Koh Tao has a couple of small temples. Little is left to see of its 1930s past as a place of political exile.

Chaweng gives Samui a proper night out. There are clubs in the lanes behind the sand, beach clubs with DJs and fire shows, and bars that run well past midnight in high season, though parts of it are generic and seedy. Lamai is rougher and Bophut is for a quiet drink by the water.

Koh Tao's evenings are smaller and looser. Young divers gather on cushions at the south end of Sairee for fire-dancing, bar crawls and themed parties, but there are no real clubs and the scene thins out in the rainy months. Both islands are a boat ride from the full moon parties at Haad Rin on Koh Phangan.

Underwater, Koh Tao has the edge. Coral gardens ring its coast, and Chumphon Pinnacle and Sail Rock draw barracuda, trevally and the odd whale shark. Its calm, warm sites make it one of the world's busiest places to learn to dive. On land, its granite boulders offer climbing, bouldering and steep viewpoint trails.

Samui's best natural sight is a boat trip away. Ang Thong National Marine Park is a group of over forty limestone islands with sheer cliffs and an emerald saltwater lake, good for kayaking and snorkelling. On Samui itself there are the Na Muang waterfalls and jungle hills, but divers based there travel north towards Koh Tao's sites.

From both islands, day trips mean island-hopping rather than sightseeing, but the trips run in opposite directions. From Samui you go out to smaller, quieter places: laid-back Koh Phangan within an hour, or a full day on Koh Tao and the Koh Nang Yuan sandbar. From Koh Tao, Koh Nang Yuan is a short hop, and the bigger trips are to larger islands with towns, temples and markets: about an hour to Phangan and about two to Samui.

Neither island has historic towns within reach. On Koh Tao, ferry timetables and monsoon seas also make leaving less flexible.

Samui handles young children far better. Choeng Mon, Bophut and Maenam have calm, shallow water, and many resorts are built around families, with large pools and children's clubs. Animal parks, a water park and boat trips to Ang Thong fill the days for older children.

Koh Tao suits children who swim confidently. The water at the north end of Sairee and in Shark Bay is gentle, and dive schools run courses for children. With toddlers it is hard going: many bays are down steep tracks, nothing is built for children, and serious medical care is a boat ride away.

Neither island is easy without your own wheels, but Samui is simpler. One ring road of about 51km links every beach, Chaweng to Bophut or Lamai takes about a quarter of an hour, and ride-hailing and hire cars add to the songthaews and bargained taxis. The island's own airport also saves a ferry crossing.

Koh Tao is small, and the walk from Sairee to Mae Haad is pleasant, but there are no buses and the roads to the eastern bays are steep and rutted, so longtail boats fill the gaps. Getting there means a ferry from Chumphon or Samui and a long travel day. Scooters are risky on both islands' hills.

Samui at least gives you a browse. The weekly walking street in Fisherman's Village has crafts and coconut-wood homeware, Lamai and Maenam have their own markets, tailors make suits, and Chaweng has a large modern mall. Koh Tao has little more than sarongs, T-shirts and dive gear along Sairee and the pier, so bring anything you need.

Koh Tao is the cheaper island for a longer stay. Simple bungalows, Thai meals and beer cost little, dive courses are among the best value anywhere, and booking a room through a dive school lowers the cost further. Imported food and drink cost more because everything comes by boat.

Samui is one of Thailand's pricier places to stay, with resort villas, expensive taxis and limited flights. It still rewards visitors who eat at the night markets, sleep in guesthouses back from the beach and stay on the north or west coast rather than in Chaweng.

Neither island is a place for serious performance. Samui has a little more entertainment aimed at visitors, with Muay Thai fight nights at small stadiums in Chaweng and Lamai and ladyboy cabaret shows. On Koh Tao, live entertainment means fire-dancing on the sand and the occasional acoustic set in a bar.

The difference is between a stay with a programme and a stay with nothing planned. On Samui the rest can be the whole point of the trip. You can book a week of supervised fasting, meditation or yoga, end the day in a resort spa, and find cheap massage on every beach in between. Quiet comes from picking your coast, because Chaweng is loud and the north and west are not.

On Koh Tao, rest fits in around the sea. A massage after a dive, a yoga class and a handful of resort spas are about the extent of it, and the small inland retreats are modest next to Samui's. The southern coves around Chalok Baan Kao and the eastern bays are properly quiet, while Sairee stays noisy late into the night.