Sanibel or Siesta Key?

Both islands have calm, shallow Gulf water that shelves gently. Siesta Key has the better sand. Siesta Beach is a wide sweep of white quartz so fine it stays cool underfoot, and Point of Rocks off Crescent Beach gives you somewhere to snorkel.

Sanibel's draw is what washes up rather than the sand. People walk the long, undeveloped stretch of Bowman's Beach at low tide after winter storms, looking for whelks and the rare junonia. Turtle Beach on Siesta Key has shells and fossilised shark teeth, but Sanibel is the island famous for shelling.

On both islands the menu is grouper sandwiches, shrimp and stone crab claws in winter, eaten on porches and decks. Siesta Key does better because of its neighbour. Downtown Sarasota and St Armands Circle are about twenty minutes away, with modern American and international cooking and a Saturday farmers' market.

On Sanibel, the breakfast spots and casual places along Periwinkle Way and on Captiva are pleasant but much alike. For a serious meal you have a longer drive across the causeway to Fort Myers, or down to Naples.

Sanibel's sights all relate to the island: the Bailey-Matthews National Shell Museum, the 1884 iron-frame lighthouse and a village of pioneer cottages. You can see them in a morning.

Siesta Key has no museums of its own, but Sarasota offers more. The Ringling combines a Baroque art collection, John Ringling's Venetian Gothic mansion and a circus museum. Add the Selby Botanical Gardens and the mid-century modern houses of the Sarasota School, and you have a day or two of sightseeing.

Siesta Key has a real, if modest, night out. Siesta Village is a few blocks of open-air bars with frozen cocktails and cover bands. It is busiest during spring break in March and April, which is worth knowing whether you want the crowd or want to avoid it. Downtown Sarasota adds rooftops and cocktail bars.

Sanibel goes quiet well before midnight. A few restaurant bars on Captiva have a guitarist on winter nights, and for anything later you drive to Fort Myers Beach.

Sanibel has the stronger wildlife on its doorstep. The 'Ding' Darling refuge takes up the bay side of the island, and on winter mornings at low tide its Wildlife Drive passes mudflats full of spoonbills, ibises and herons. The mangrove tunnels of Tarpon Bay and Commodore Creek are a short paddle from the beach.

Around Siesta Key the best of it is further off. The mangrove tunnels are on Lido Key, and Myakka River State Park, with its canopy walkway and alligators, is about forty minutes inland. In its favour, sea turtles nest on its beaches from May to October.

Siesta Key sits better for outings by road. St Petersburg and its Dalí Museum are about an hour north across the Sunshine Skyway, and Tampa's Ybor City, Tarpon Springs, Anna Maria Island and 1920s Venice are all within a day.

Sanibel's best outings go by boat: to Cabbage Key and its old inn, or to the empty beaches of Cayo Costa. By road there is the Edison and Ford Winter Estates in Fort Myers and polished Naples to the south. Both islands can reach Boca Grande, Sanibel by water and Siesta Key by car.

Both are easy places for a slow beach vacation with young children, with shallow, gentle water and rented condos and cottages that give everyone room. Siesta Beach has more laid on: lifeguards, a playground and showers, and Mote Marine's aquarium on Lido Key nearby.

Sanibel keeps children busy in a different way. They can spend hours shelling, cycle on the safe flat paths, spot alligators in the refuge and use the touch tanks at the shell museum. Teenagers may tire of it sooner than of Siesta Key.

Both are reached by car across a bridge or causeway, and both get winter traffic jams and full beach car parks. Once you are there, they work differently. Sanibel is made for cycling, with flat shared-use paths linking the beaches, the refuge and the shops.

Siesta Key is a narrow strip about 13km long, with a walkable village and a free trolley along the main road. A beach-only stay works without a car. Anything on the mainland still needs one.

Siesta Village sells only beach basics, but St Armands Circle, about twenty minutes away, has upscale boutiques and jewellers, and Sarasota's Main Street has bookshops and galleries. That gives Siesta Key the edge. On Sanibel, the plazas along Periwinkle Way and in Captiva village hold shell shops, resort clothing and coastal art, which is pleasant for an hour, and anything more means the malls on the mainland.

Neither island is cheap, and both are at their most expensive from January to April. Sanibel costs more. Height limits keep the supply of beds small, there are no budget hotels, and you need a car. Many visitors save by staying in Fort Myers.

Siesta Key is somewhat easier on the budget. The free trolley means a beach stay can manage without a car, self-catering in a condo is the norm, and summer is noticeably cheaper if you can live with the heat and storms.

The gap here is wide, and it comes from Sarasota. A short drive from Siesta Key, it has a resident opera company in a restored 1920s opera house, the Asolo Repertory Theatre at the Ringling, a ballet company, a symphony orchestra and the Van Wezel hall for touring musicals. On the island itself there is the Sunday sunset drum circle.

Sanibel's winter season has chamber concerts and small-scale theatre. For Broadway tours you drive to Fort Myers.

The two are equally restful, and neither has a bathing or retreat tradition. What helps you unwind comes from the pace rather than from spas. On Sanibel, the absence of high-rises and the slow roads make the whole island quiet, and the larger resorts there and on Captiva have full-service spas.

On Siesta Key, the quiet is concentrated at the south end around Crescent and Turtle beaches, away from the village. Wellness there means beach yoga, a few day spas and the pool of your rented condo.