London or Saint-Malo?

Both look onto the same cold Channel, so neither is for warm swimming, but Saint-Malo has its beaches on the doorstep. The Grand Plage du Sillon runs for about 3km from the ramparts, the beaches roughly double in size at low tide, and the walled pool at Bon-Secours means you can swim whatever the tide is doing.

From London the seaside is a day out: Brighton's pebble beach is about an hour away by train, and Margate and Whitstable have sandier shores. In the city itself people swim in lidos and the Hampstead Heath ponds, which are good on a hot day but are no beach.

London has far more range. You can eat Bangladeshi food on Brick Lane, Punjabi cooking in Southall, Turkish grills in Green Lanes and Caribbean food south of the river. Add Borough Market, the weekend food markets and plenty of fine dining, though you will rarely eat cheaply.

Saint-Malo does one thing well: Breton cooking from the sea and the buckwheat field. That means galettes with dry cider, oysters eaten by the harbour wall at Cancale, and mussels and seafood platters. The crêperies inside the walls vary a lot, so look for better meals in Saint-Servan or towards Cancale, and expect little international food.

London can fill weeks. The British Museum, the National Gallery, the V&A and the two Tates are each worth a day, and most cost nothing to enter. Around them are landmarks from the Tower of London and Westminster Abbey to Wren's St Paul's and the Royal Observatory at Greenwich.

Saint-Malo's main sight is the town itself. You can walk the full circuit of the ramparts, see the cathedral's stained glass and the castle keep, and cross at low tide to Chateaubriand's grave on the Grand Bé. Much of the town was rebuilt after 1944 and there are few serious museums, so a day or two is enough.

In London you choose a night out by district: late cocktail bars and the long-established gay scene in Soho, warehouse parties in Hackney and Dalston, big venues and rooftop bars in Brixton, and pubs everywhere. Saint-Malo is for a drink after dinner in the pubs and wine bars inside the walls. The streets are livelier with sailors in July and August, the town goes quiet early the rest of the year, and the nearest clubs are in Rennes.

Saint-Malo has the more striking outdoors. The GR34 coastal path runs east to Pointe du Grouin and west to the cliffs of Cap Fréhel. Tides of up to about 13m uncover huge sand flats, and the Rance estuary and the bay are good for kayaking, sailing and kitesurfing.

London's green space is mostly inside the city: the deer of Richmond Park, the views from Hampstead Heath, Kew Gardens and the Thames Path. The Chilterns and the South Downs are about an hour away. Neither has mountains, so Saint-Malo pulls ahead on its sea and tides.

London's rail network gives it the longer list, all without a car. Oxford and Cambridge are about an hour away, and Windsor, Bath, Canterbury and Stonehenge are all within reach, enough for several very different days out in one week.

Saint-Malo's big outing is Mont-Saint-Michel, about an hour's drive away. Medieval Dinan, Dinard across the estuary and Rennes by train are good shorter trips, and summer ferries go to Jersey. Most of the trips around Saint-Malo are easier with a car.

London gives children more to do. The dinosaur halls at the Natural History Museum, the Science Museum and the Young V&A are all free. Add London Zoo, the pirate ship in the Diana Memorial Playground, and Legoland and the Harry Potter studio tour outside the city. The drawback is size: journeys are long, and Tube stairs are hard with a buggy.

Saint-Malo makes a simpler seaside week of sandcastles, jumps into the Bon-Secours pool, pirate stories on the ramparts and the Grand Aquarium for wet days. The sea is cold for small swimmers, the tides come in fast, and there are no theme parks nearby.

You never need a car in London. The Tube, the Elizabeth line, buses and riverboats all take the same contactless payment, though the city is spread out and rush hour is packed.

Saint-Malo is small enough to walk from the walls along the promenade to Paramé, and buses and the Dinard boat cover the rest of the town. Reaching Mont-Saint-Michel or the coastal villages usually means driving or joining a tour, because buses there are thin.

For London, shopping is a reason to go. It has the luxury houses of Bond Street, bespoke tailoring on Savile Row, landmark stores such as Liberty and Fortnum & Mason, and weekend markets from Portobello Road to Columbia Road. In Saint-Malo you shop for Breton things: striped sailor tops, salted-butter caramel, cider, faience and good cheese. For fashion you would go to Rennes.

Neither is cheap, but Saint-Malo is easier on the budget. Summer rooms near the walls cost a lot and sell out early. On the other hand, a galette with cider is inexpensive, the set lunch menus are fair, and prices drop in spring and autumn.

In London almost everything costs more, from small hotel rooms to theatre seats and fares. What helps is that the national museums, the parks and many of the best walks are free.

London has an enormous stage. There are West End musicals and plays, the National Theatre and the Globe, opera and ballet at Covent Garden, five major orchestras and the Proms at the Royal Albert Hall, so there is a serious choice every night.

Saint-Malo has a modest theatre season and the occasional concert in the cathedral. Most of its cultural life happens at festivals such as La Route du Rock and Étonnants Voyageurs, and on most other nights there is little on.

London is not a city for unwinding. It has historic public Turkish baths with steam rooms and plunge pools, in Bayswater and at Ironmonger Row in Islington, and full spas in the grand Mayfair and Knightsbridge hotels. The pace stays fast, though, and for a proper spa you take the train to Bath.

Saint-Malo is the more restful of the two, a classic French resort for thalassotherapy, the seawater cure. The large centre on the Sillon beach has heated seawater pools and hydrotherapy. Outside August, a session goes well with a long walk on the empty sand at low tide.