Key Largo or Siesta Key?

These are two very different shorelines. Key Largo is built on old coral, so swimming means small man-made patches of sand in John Pennekamp, a calm resort lagoon or a dive boat out to the reef, with seagrass and mangroves at the water's edge.

Siesta Key has exactly what Key Largo lacks. Siesta Beach is a broad sweep of fine white quartz sand shelving gently into the Gulf, and further south there are quieter Crescent Beach and shell-hunting at Turtle Beach. For lying on the sand and wading in, it wins easily. Key Largo's one advantage is a sea that stays warmer in winter.

Both islands eat the same way: fresh fish grilled, blackened or in sandwiches at open-air places by the water, with stone crab in winter. Key Largo adds conch fritters and Key lime pie, but for anything beyond the seafood formula you face the long drive to Miami.

Siesta Key comes out a little ahead because Sarasota is only twenty minutes away. Downtown and St Armands Circle have modern American and international restaurants and a Saturday farmers' market, so a varied dinner is an easy evening out.

Neither island has real sights of its own, so the difference is what lies nearby. From Key Largo, culture means the African Queen boat at a marina, the Christ of the Abyss statue on the reef and a small diving museum in Islamorada. That is thin for anyone who wants more than the sea.

Siesta Key has Sarasota across the bridge, and the Ringling alone fills a day: Baroque painting including Rubens canvases, the Ca' d'Zan mansion on the bay and the circus museum. Add the Selby botanical gardens and the mid-century Sarasota School houses, and there is enough for a couple of days away from the sand.

Both are places for a sunset drink rather than a night out. Key Largo's tiki bars on the bay side, often with a guitarist playing, empty early because the dive boats leave at dawn.

Siesta Village has more going on: a few blocks of open-air bars with cover bands, and a spring-break crowd in March and April. Sarasota's cocktail bars and rooftops are close by for a more grown-up evening. Neither has clubs.

This is where Key Largo pulls clearly ahead. John Pennekamp protects part of the only living coral barrier reef in the continental United States, with Molasses and French Reef for snorkellers and the sunken Spiegel Grove for deeper dives. The bay side adds mangrove paddling, flats fishing and the edge of the Everglades.

Siesta Key's nature is gentler and closer to shore. You can see dolphins from the beach, manatees in the bays in the cooler months and nesting turtles in summer. There are mangrove tunnels to paddle on Lido Key, and alligators at Myakka River State Park inland. It is good for a kayak and a wildlife walk, but there is no reef to compare.

Key Largo has one strong outing, Miami, about an hour and a half north. Closer in there are Coral Castle in Homestead and the sea turtle hospital at Marathon, while Key West is really too far to see in a day.

Siesta Key sits on a coast with options in both directions. St Petersburg and its Dalí Museum are across the Sunshine Skyway, with Ybor City in Tampa and the sponge docks of Tarpon Springs further on, and Anna Maria Island, Venice and Boca Grande nearer to hand. None is first rank, but the choice is wider.

With young children, Siesta Key is the easier choice. The main beach has lifeguards, a playground and showers, the sand slopes so gently that toddlers can paddle a long way out, and rented condos come with kitchens and pools. Mote Marine Aquarium on Lido Key is the main outing off the sand.

Key Largo suits older children who can snorkel. They can look down on the reef from a glass-bottom boat, visit the small aquarium at John Pennekamp or join a dolphin programme. Its tiny beaches give little ones far less to do.

On both you will want a car for anything beyond your base, so the difference is in layout. Key Largo stretches along some 30km of the Overseas Highway, with places addressed by mile marker and few pavements. Dive boats do collect from marinas near most hotels.

Siesta Key is compact, about 13km long, with a walkable village, easy cycling and a free trolley, so a beach-only stay can manage without a car. The catch is bridge traffic and full car parks in the winter season.

Nobody goes to either island to shop, but Siesta Key does better. Key Largo has souvenir and beachwear shops, a few galleries and very good dive and fishing shops, and little else closer than Florida City or Miami.

From Siesta Key, the boutiques and jewellers of St Armands Circle and the independent shops on Sarasota's Main Street are about twenty minutes away, enough for a pleasant afternoon's browsing.

Both islands are expensive in their winter high season, so the real question is where the money goes. In Key Largo it goes on rooms by the water and on dive, snorkel and fishing charters. Staying on the mainland in Homestead or Florida City cuts costs sharply, at the price of a daily drive.

On Siesta Key the beach and the trolley are free, and cooking in a rented condo keeps spending under control. Summer is cheaper if you can put up with the heat and storms.

Key Largo has almost nothing here beyond a guitarist on a bar deck at sunset and a small community theatre in the Upper Keys. For concerts or theatre you have to go to Miami.

Through Sarasota, Siesta Key has an arts life well beyond its size: opera in a restored 1920s opera house, the Asolo Repertory Theatre at the Ringling, ballet, a symphony orchestra and touring shows at the Van Wezel hall. The season runs from autumn to spring, and the Sunday drum circle on the beach is a looser local ritual.

Key Largo's idea of rest is a bay-side resort: a hammock, a pool, a quiet lagoon and, at the larger places, a full spa with massage and treatments, plus yoga on a dock. Days start early on a boat and end by the water. The highway runs through the middle of the island, though, so pick somewhere on the bay side, away from the road.

Siesta Key has no resort spas to match, only a handful of day spas, massage studios and beach yoga classes. Its calm comes from the beach itself, with morning walks and long afternoons on the sand, and it is quietest at the south end around Crescent and Turtle beaches. Neither island has a bathing tradition.