Santa Monica or Sun Valley?

This is the clearest difference between the two places. Santa Monica has a broad band of pale sand along the whole town, with lifeguard towers and the pier at its centre. Quieter, more scenic coves lie further along at Point Dume and El Matador. The Pacific stays cool, so many people paddle rather than swim properly.

Sun Valley is in the mountains of Idaho, hundreds of kilometres from any coast. The closest thing to a beach day is the sandy shore of Redfish Lake under the Sawtooths in midsummer, and even then the water is cold.

Santa Monica eats better and with more variety. The Wednesday farmers market sets the tone for seasonal Californian cooking, with seafood, brunch and solid Italian and Japanese restaurants along Main Street and Montana Avenue. Venice next door adds more casual places. The cheap immigrant food Los Angeles is known for is a long drive east.

Sun Valley's dining is concentrated in Ketchum. A few ambitious kitchens cook trout, lamb and elk, alongside après-ski grills and pizza. The choice is small and rarely cheap, and some places cut back in the quiet seasons.

Neither is a sightseeing destination, but Santa Monica has more to offer. There is the pier and its carousel at the end of Route 66, the galleries of Bergamot Station and some fine Art Deco buildings. The Getty Center and the Getty Villa are a short drive away and add serious art and antiquities.

Sun Valley's interest is mostly historical: the 1936 Lodge built by the Union Pacific railroad, the world's first chairlifts and Hemingway's grave in Ketchum. An afternoon covers it.

Neither town is built for clubbing, but Santa Monica gives you much more to do after dark. Around the Third Street Promenade you can choose between sports bars, wine bars and hotel rooftops looking out over the ocean. Main Street has British-style pubs, and in Venice next door the bars are scruffier and draw more residents than visitors. The town still goes quiet fairly early, and if you want to dance you face a long taxi ride to Hollywood or West Hollywood.

Sun Valley has a small fraction of that. After skiing, people fill a few Ketchum saloons, wine bars and hotel lounges, and a band may play on a busy winter weekend, but there are no clubs at all. In the spring and autumn lulls, the bars are largely empty by mid-evening.

Both have good outdoor options, but Sun Valley has more serious terrain. Bald Mountain drops straight into Ketchum with long, uninterrupted runs. In summer the valley has dense singletrack, fly fishing on the Big Wood River and Silver Creek, rafting on the Salmon River, and the alpine lakes and climbing of the Sawtooths. In autumn the aspens turn gold.

Santa Monica's outdoors are gentler and available every day of the year. There are chaparral canyon trails in Topanga and Temescal, about 35km of beach path for cycling, surf lessons, and whales and dolphins offshore.

Santa Monica has far more places to visit for a day, though traffic decides how far you get. Options include Santa Barbara's mission and wine country, Ojai, the coves of Laguna Beach, Catalina Island by ferry, and the Huntington gardens in Pasadena.

Around Sun Valley the country is wild rather than settled. Hailey's small main street, the log-cabin outpost of Stanley and Shoshone Falls about two hours south are about the extent of it. Most outings head into the mountains rather than to other towns.

Santa Monica is the easier choice with children. It has a wide, lifeguarded beach, a Ferris wheel and roller coaster on the pier, a small aquarium of local sea life, playgrounds along the sand and mild weather all year. Theme parks are a long drive, though.

Sun Valley suits families who ski, with a separate beginners' hill and a good ski school. In summer there are horse rides and an outdoor ice rink. With nothing built specifically for children, cold midwinter days and the altitude, it is harder going with toddlers and with teenagers who don't ski.

Both places are easy within their own limits and need a car beyond them, but Santa Monica has the edge. Its walkable centre and blue buses cover the town, and the E Line light rail runs all the way to downtown Los Angeles. The Getty, Malibu and Hollywood still mean jammed freeways.

Sun Valley has a free bus linking Ketchum, the village and the lift bases, so a ski week without driving is easy. Trailheads, hot springs and the Sawtooths have no transit, and the larger airports are a long drive over winter roads.

Santa Monica has much more range. Chains and flagships line the Third Street Promenade, Montana Avenue has upmarket boutiques and children's shops, Main Street has surf shops and independents, and Abbot Kinney in Venice has design-led stores. Rodeo Drive is a short but often slow drive inland.

Sun Valley's shopping fits into a few blocks of Ketchum. It has western and fine art galleries, very good ski and fly-fishing shops, and cashmere and home decor boutiques aimed at second-home owners. Prices are high and the choice is narrow.

Both are expensive, and there is little to choose between them on price. They differ in where the costs fall. In Santa Monica, oceanfront hotels, restaurant bills with tax and tip, and car hire add up, but the beach, trails and bike path are free and the light rail is cheap.

In Sun Valley, the second-home community sets prices and lift access adds to the bill. Budget lodging is scarce, so the main ways to save are staying down the valley or coming in spring or autumn.

The difference here is timing more than scale. Santa Monica has performances all year: chamber music, dance and theatre at the Broad Stage, small folk and acoustic rooms, and free summer concerts on the pier. The Hollywood Bowl and Walt Disney Concert Hall are across the city, a long drive at rush hour.

Sun Valley saves its performances for summer. For several weeks a professional orchestra gives free concerts in the pavilion by the Lodge while people picnic on the lawn, and star skaters perform on the outdoor rink. The rest of the year there is very little.

The two are broadly level for relaxation, but the experience is very different. Santa Monica's version is active and social: yoga on the sand, juice bars, morning runs by the ocean, polished hotel spas and sunset over the bay.

Sun Valley offers warm water and quiet. There are steaming outdoor pools under falling snow, resort spas and rough natural hot springs beside the creek up the Warm Springs valley. Evenings are early and the mountain air does the rest.