Key Largo or Sanibel?

Sanibel wins this easily. Nearly its whole Gulf shore is sand shelving into calm water, from the long, undeveloped stretch of Bowman's Beach to Lighthouse Beach at the eastern tip, and the shelling turns a walk along the tideline into a pastime of its own.

Key Largo is built on old coral rather than sand, so it has almost no natural beach, just small man-made patches in John Pennekamp, the town parks and the resort lagoons. Its sea is warmer in winter, though, and its best swimming is from a boat over the reef. It suits people who want to be in the sea rather than lying beside it.

Neither island is a reason to travel for food, and both serve much the same meal: local fish grilled, blackened or in a sandwich, with stone crab in winter. Key Largo eats on open-air docks and canal sides along the Overseas Highway, with Key lime pie as its signature, while Sanibel leans on breakfast places and casual porch restaurants along Periwinkle Way and on Captiva. For more range, Key Largo looks north to Miami and Sanibel to Fort Myers and Naples.

Sanibel has more to see, though it is still only a morning's worth. There is the Bailey-Matthews National Shell Museum, the only one of its kind in the country, the iron lighthouse from 1884 and a village of pioneer cottages at the historical museum.

Key Largo has even less on land. The African Queen at its marina and the diving history museum down in Islamorada are curiosities, and its best-known sight, the bronze Christ of the Abyss, can only be seen with a mask on.

Neither is a place for a big night out, but Key Largo has slightly more going on. Its bay-side tiki bars draw a crowd for sunset with a guitarist, and a few stay busy late at weekends. Sanibel's evenings are dinner and an early night, livened only by a handful of restaurant bars on Captiva in winter. For anything bigger, Key Largo looks to Miami and Sanibel to Fort Myers.

Key Largo has the stronger outdoors, and most of it is offshore. John Pennekamp protects part of the only living coral barrier reef in the continental United States, with shallow Molasses and French Reef for snorkellers and the Spiegel Grove for deeper dives. Behind the island lie mangrove channels for paddling, flats fishing in Florida Bay and the Everglades across the water.

Sanibel's nature is wildlife at the water's edge. The Ding Darling refuge puts spoonbills, pelicans and alligators a short ride away, and Tarpon Bay has mangrove tunnels for kayaking, but there is no reef to dive. Both islands are completely flat, so neither suits walkers looking for hills.

Both have a modest choice of outings, but they point in different directions. Key Largo's best day out is a city one: Miami, about an hour and a half north, for South Beach and Little Havana, with Islamorada, the turtle hospital at Marathon and Coral Castle closer by. Key West is too far to manage there and back in a day.

Sanibel's outings are quieter and often by water: boats to Cabbage Key, the empty beaches of Cayo Costa or old Boca Grande. On land there are the Edison and Ford Winter Estates in Fort Myers and the old town of Naples to the south.

Sanibel is the easier family island. Shallow Gulf water, hours of shelling, flat bike paths and the touch tanks at the shell museum suit even small children, and rented condos give room to spread out. Teenagers may run short of things to do after a few days.

Key Largo works better once children can snorkel, when glass-bottom boats over the reef, dolphin programmes and the marine park in Islamorada become real treats. With tiny beaches and everything a drive away, it is harder going with toddlers.

Both need a car, but the days feel different. Key Largo stretches along some 30km of the Overseas Highway with few pavements, so every outing means driving or catching a dive boat from the marina. Sanibel is compact, and once you are over the causeway the shared-use paths make a bicycle the nicest way between beach, refuge and shops, though winter traffic clogs Periwinkle Way.

Sanibel is good for an hour or two of browsing, with shell shops, resort-wear boutiques and galleries of coastal art in the plazas along Periwinkle Way and in Captiva village. Key Largo has less: souvenirs, beachwear and well-stocked dive and fishing shops, which are useful for gear but not for an afternoon out. From either island, serious shopping means a drive to the mainland malls.

Neither island is cheap, but Key Largo is a little kinder on the budget. Sanibel has no budget hotels, its height limits keep rooms scarce, and winter cottages book up far ahead at high-season rates.

In Key Largo, waterfront rooms and charters soon add up too, but staying in Homestead or Florida City and driving in cuts costs sharply. On Sanibel, the savings come from a rental with a kitchen and from beaches and bike paths that cost little to enjoy.

Sanibel has a small but real winter season that Key Largo lacks, with chamber concerts, talks and amateur and touring theatre, plus acoustic sets in a few restaurants. Key Largo offers little beyond a guitarist on a sunset deck and a small community theatre in the Upper Keys. For anything larger, Sanibel looks to the performing arts hall in Fort Myers and Key Largo to Miami.

Both islands are restful, with resort spas and yoga but no bathing tradition of their own. Where they differ is in where the calm comes from. On Key Largo it comes from early starts on the water and a hammock at a bay-side resort, as long as you stay away from the highway through the middle. On Sanibel it comes from the whole island, with no high-rises, slow roads and days that follow the tides.