Buenos Aires or Rio de Janeiro?

This is the widest gap between the two. Buenos Aires has no swimming at all: the Río de la Plata is silty and polluted, the Costanera is for strolling, and the Atlantic resorts such as Mar del Plata need a separate stay of several days.

In Rio the beach is the heart of the city. Copacabana, Ipanema, Leblon and Arpoador are a short walk from most hotels, the ocean is warm enough to swim much of the year, and Prainha and Grumari offer cleaner water and surf further west. Mind the currents on the open beaches and skip the polluted water inside Guanabara Bay.

Buenos Aires is the stronger place to eat. The parrilla is an institution, with cuts, offal and chorizo off the wood grill, and the Italian heritage brings thick pizza, fresh pasta and excellent ice cream. Grand coffee houses such as Café Tortoni and Palermo's modern kitchens and wine bars add depth, even if the range narrows beyond beef, pasta and pastry.

Rio eats more casually, in tiled botecos over cold chopp and shared plates, with feijoada on Saturdays, fruit juice bars on every corner and coconut water on the sand. It is enjoyable and cheap at lunch, but the cooking rarely becomes the reason for the trip.

The two are well matched, but they reward different kinds of sightseeing. In Buenos Aires the architecture is the attraction: Parisian boulevards from the Casa Rosada along Avenida de Mayo to the Congress, the marble mausoleums of Recoleta Cemetery, and stronger art collections at the Museo Nacional de Bellas Artes and MALBA.

Rio's headline sights are tied to its landscape, with Christ the Redeemer on Corcovado and the cable car up Sugarloaf. Its old centre holds quieter treasures such as the gilded São Bento monastery and the Real Gabinete Português de Leitura, and football fans will want Maracanã, though the sights fill a few days rather than a week.

Both cities go out late, but Buenos Aires takes it further. Bars fill at midnight, clubs barely start before two and run past sunrise, Palermo Soho and Hollywood hide cocktail bars behind unmarked doors, and electronic music is taken seriously. The milongas, where locals dance tango into the small hours, give the city a kind of night out Rio has no equivalent for.

Rio's nights happen more on the street: crowds under the Lapa Arches with caipirinhas from stalls, pavement tables in Botafogo and Humaitá, and baile funk for a harder edge. Carnival and the blocos before it are unmatched, but between districts after dark you will be taking taxis.

Rio is in another league here. Tijuca National Park covers the mountains in the middle of the city with rainforest, waterfalls and marmosets, short steep hikes reach views over the bay, and you can surf, paddleboard on the lagoon or hang-glide onto the beach at São Conrado. The Serra dos Órgãos offers serious mountain walking about two hours inland.

Buenos Aires is flat pampas. Its outdoors means jogging loops in the Bosques de Palermo, birdwatching in the Costanera Sur wetland and a kayak through the channels of the Paraná Delta, which is pleasant but gentle.

Buenos Aires has the better spread of easy days out. The ferry to Colonia del Sacramento puts a cobbled Uruguayan old town within an easy return trip, Tigre and the delta are an hour by train, and San Antonio de Areco shows the gaucho world of saddlers and estancias.

Rio's best outing is Petrópolis, the imperial summer town in the cool hills, with Niterói and its Niemeyer museum a ferry ride across the bay. Its real treasures, Paraty, Búzios and Ilha Grande, lie three hours or more away and need an overnight stay.

Neither is an easy family city, and both welcome children warmly. Buenos Aires builds a day around the Palermo parks, with pedal boats, playgrounds and the planetarium, plus the Museo de los Niños for rainy days, though very late dinners and long distances tire younger children.

In Rio the beach does the work, with calmer water at Leme and Praia Vermelha, and the Sugarloaf cable car and Corcovado railway make easy thrills. Summer heat and the need to watch belongings are the main strains.

A flat, regular grid makes Buenos Aires simple to walk within a neighbourhood, especially under the plane trees of Palermo and Recoleta, though San Telmo, the centre and Palermo lie a long way apart. The Subte covers the centre and Palermo, and the colectivo buses reach everywhere day and night on the same card, so few trips need a taxi.

Rio's metro is clean and follows exactly the right line, from the centre through Botafogo to Copacabana, Ipanema and Leblon. Step off that corridor, to Santa Teresa, Tijuca or the western beaches, and you depend on taxis and ride-hailing, with slow traffic through the tunnels.

Buenos Aires gives shoppers more to look for. Leather jackets, bags and shoes from the workshops of Villa Crespo are the classic buy, Palermo Soho is good for Argentine designers, and the Sunday fair along Calle Defensa in San Telmo runs for blocks of antiques and tango records.

Rio shops for the beach: swimwear, flip-flops and linen in Ipanema and Leblon, crafts at the Sunday hippie fair in Praça General Osório, and Carnival costumes in the crowded Saara streets. Beyond that there is little local craft to seek out.

Costs are broadly similar, but they fall in different places. In Buenos Aires transport, parrillas and wine are cheap and hotels cost less than in a European capital, while Palermo and Puerto Madero charge European prices and the long history of currency swings makes every trip a little unpredictable.

In Rio the best things, the beaches, the promenade and the forest trails, are free and everyday food and transport cost little. Accommodation is the catch: rooms near the sand are dear for what they offer, and rates jump steeply at Carnival and New Year.

The Teatro Colón, one of the world's great opera houses, anchors a stage scene in Buenos Aires that runs from the playhouses of Avenida Corrientes to one of the largest independent theatre circuits in the Spanish-speaking world. Tango ties it together, from polished dinner shows to milongas with live orchestras in neighbourhood halls.

Samba is Rio's answer, heard most nights in Lapa's old houses, at rodas in squares and at samba school rehearsals before Carnival, with bossa nova in small venues. Beyond the Theatro Municipal the formal scene is thin, and theatre is in Portuguese, so the music carries the city.

Neither city is built for unwinding, and neither has a bathing tradition, so wellness means hotel spas in both. In Buenos Aires calm comes from lingering over coffee and the long sobremesa after a meal, and from the empty streets on a Sunday morning.

Rio relaxes outdoors instead, with yoga on the sand, an evening walk along the promenade or the lagoon, and the cooler hills of Santa Teresa as an escape from the heat.