Ios or Santorini?

This is Ios's strongest suit. Its beaches are golden sand: Mylopotas is long and sociable, with water sports and sunbeds below Chora, Manganari's coves in the south are sheltered and very shallow, and the bays on the east and north coasts are wilder and emptier for anyone with a scooter.

Santorini's swimming is volcanic and different in kind. Kamari and the long strip from Perissa to Perivolos have black sand and fine pebble that gets very hot underfoot, set against dramatic coloured cliffs. They are good to look at but less comfortable to lie on, and the caldera villages where most people stay have no beach at all. The water is equally warm on both islands, and the meltemi roughs up the exposed coasts of each.

Santorini eats far better, and much of the difference grows out of its volcanic soil. It has yellow-pea fava, cherry-tomato fritters, white aubergines and capers, and above all wine: basket-trained Assyrtiko vines and sweet Vinsanto, tasted at wineries around Pyrgos and Megalochori. The best meals are in the inland villages and at the fish tavernas of Ammoudi Bay rather than on the sunset rim.

Ios has honest harbour tavernas in Yialos and its own tangy skotyri cheese. In high summer, though, many of its kitchens feed the bar crowd with gyros and pizza, and a good dinner takes some looking for.

Santorini is well ahead. Akrotiri, a Bronze Age town preserved under volcanic ash with multi-storey houses and fine wall paintings, is one of the great sites of the Aegean. Ancient Thera, the Museum of Prehistoric Thera in Fira, the cliff-hung villages of Oia and Imerovigli and the medieval kastelli of Pyrgos and Emporio add enough for several days.

On Ios the sightseeing is modest. There are Chora's windmills and white chapels, the Early Cycladic walls at Skarkos, the spot at Plakotos that tradition calls Homer's tomb and the ruined castle of Paleokastro. It makes a pleasant few hours between beach days.

Ios is the party island of the pair. In summer, Chora's old lanes and main square pack dozens of small bars and a few clubs together, and the crowd drifts from one doorway to the next until dawn. Mylopotas adds afternoon beach parties that run into the evening. It is young, loud and cheap rather than polished.

Santorini's evenings are built around the sunset, a long dinner and a drink with a caldera view, which suits couples more than night owls. Fira has a few lanes of cocktail bars and clubs that run late in high summer, and Oia goes quiet after dinner.

Ios is good walking country but has nothing to match its neighbour. Its bare, terraced hills climb to Profitis Ilias, a modest summit rather than a real mountain. Old mule tracks lead to the windmills, remote chapels and northern bays, and boat trips reach sea caves and coves that no road gets to. When the summer wind blows, windsurfers head for the open bay at Mylopotas.

Santorini's landscape is far more dramatic. The island is the rim of a drowned volcano, with cliffs of layered ash and lava dropping more than 300m into the caldera. The cliff-top path from Fira through Imerovigli to Oia runs for about 10km with the caldera beside you the whole way, and boats cross to climb the black lava cone of Nea Kameni. Sailing around the caldera and diving along its underwater walls add to it.

Both islands depend entirely on ferries, which run well in summer and thin out sharply in spring and autumn, so the options are broadly similar. The difference is where the boats lead. From Ios, the best day out is Santorini itself, with Naxos's Venetian old town a little further and the quiet cliff villages of Sikinos and Folegandros as gentler outings.

Santorini has a day trip inside its own caldera: small boats cross to Thirasia, a fragment of the old rim with almost no visitors. It also has the high-speed crossing to Heraklion, which puts Knossos on Crete within a long day. All of this runs through the single port at Athinios, down a switchback road below the cliffs.

Neither island is built for children, and they are about equal here. Both work as a beach base as long as you avoid the clifftop or party areas. On Ios, that means the shallow sandy coves of Manganari, but in July and August the island belongs to a young party crowd whose noise carries late, so June or September is wiser.

Santorini has no comparable party crowd, but its caldera villages are all steps and unfenced terraces, and many cliff hotels don't take young children. Kamari and Perissa are the answer, with flat promenades, gently shelving water and a small water park. The boat to the volcano and a visit to Akrotiri also give older children more to see than anything on Ios.

Ios is the easier island to move around. Its main triangle of the port at Yialos, Chora and Mylopotas is compact, linked by a frequent summer bus and an old stepped path you can walk. Only the far eastern and northern bays need a hired scooter or car. The catch is arrival: with no airport, everyone comes by ferry.

Santorini has an airport, but once on the island it takes patience. Every bus goes through Fira, so crossing the island usually means changing there on a crowded bus, and narrow roads, tour coaches and scarce parking in Oia slow down anyone driving. Fira is the most practical base without a car.

Neither island is a shopping destination, but Santorini has more worth buying. The lanes of Oia and Fira have jewellers, galleries of island painters and photographers, linen and ceramics. The best things to take home are edible: Assyrtiko, Vinsanto, capers and fava from the island itself.

Shopping on Ios runs to beachwear, sandals, jewellery and souvenirs in Chora's evening lanes, plus a few boutiques of Greek-made clothes. It covers a forgotten sundress and not much more.

Ios is much the cheaper island. Its backpacker past still shows in the hostels, campsites and simple rooms around Mylopotas and Chora, the grill food and the bars' drinks offers. Only comfortable rooms in July and August, and the ferry fares, push the cost up.

On Santorini the price is driven by the view. Cave suites with plunge pools on the rim at Oia, Imerovigli and Fira are among the most expensive rooms in Greece, and sunset dinners and boat trips cost accordingly. Staying in Kamari or Perissa and taking the bus up for the sunset brings the cost down, though not to Ios levels.

Ios has very little to go and see. The open-air Odysseas Elytis theatre by the windmills in Chora puts on the odd summer concert, and saints' day festivals bring traditional music and dancing to the villages. Beyond that, the island's music is whatever the bars are playing.

Santorini offers slightly more. Its main event is a long-running international classical music festival in Fira at the end of summer, and there are occasional concerts in restored mansions and old wineries. That is still not a reason to choose it, because neither island has a regular season of plays or concerts to plan a trip around.

Santorini is the better place to unwind, mainly because of its cave hotels. Whitewashed suites at Imerovigli and Oia come with heated plunge pools, spa treatments and long, quiet days facing the caldera, and boats stop off Palea Kameni for a swim in warm, rust-coloured spring water. Imerovigli and the wine villages of Pyrgos and Megalochori stay peaceful even when Oia is crowded.

Ios runs on late nights in high summer. A calm stay there means June or September, a hotel near Manganari or a quiet bay, and the massages and yoga a few boutique places offer.