See how the leaders in short-term apartments stack up.

Airbnb

Who They Are: This pioneer of the sharing economy lets owners rent everything from tree houses to castles, but apartments are still its mainstay.

Inventory: 500,000 houses and apartments

Best In: Barcelona, New York City, Paris.

Stay Here: A sunset-facing loft in Los Angeles’s hip Downtown arts district, with pool and gym access ($188).

FlipKey

Who They Are: On this TripAdvisor–owned peer-to-peer site, renters can post photos and link to Facebook to see if any friends have added reviews.

Inventory: 240,000 houses and apartments

Best In: Barcelona, Paris, Rome.

Stay Here: A two-bedroom in Barcelona’s Sants area, with a well-stocked kitchen and washer/dryer ($214).

Guest Apartment Services Paris

Who They Are: This Paris-only site offers a luxury-hotel experience: concierge, maid service, and limo pickups upon arrival.

Inventory: 60 apartments

Best In: Île St.-Louis and the Notre Dame district.

Stay Here: A two-bedroom flat in the Fifth with a terrace overlooking the river, walking distance from the Louvre ($570).

HomeAway

Who They Are: The company has been buying up rental sites in Australia, New Zealand, and Asia. It also has a new luxury-rental micro-site.

Inventory: 545,000 houses and apartments

Best In: Barcelona, Florence, New York City, Paris.

Stay Here: A three-bedroom loft in downtown Manhattan, set right above Great Jones Spa ($750).

Italy Perfect

Who They Are: A family business whose owners—a pair of sisters—personally inspect each listing.

Inventory: 120 houses and apartments

Best In: Florence, Rome, Venice.

9Flats

Who They Are: Europe’s answer to Airbnb, this German company has instant, last-minute booking (up to 8 p.m. the night before).

Inventory: 104,135, nearly all apartments

Best In: Barcelona, Berlin, London, Rome.

Oasis Collections

Who They Are: A South America–focused group that handpicks its properties and is known for nightlife perks such as entry to private clubs.

Inventory: 700, mostly urban apartments

Best In: Bogotá, Colombia; Buenos Aires; São Paulo.

Stay Here: A stylish one-bedroom with an outdoor deck in B.A.’s leafy Palermo Hollywood ($102).

Onefinestay

Who They Are: Its curated selection of high-end apartments all come with an iPhone full of insider tips on where to shop, eat, and drink.

Inventory: 1,500, about half apartments

Best In: London, Los Angeles, New York City, Paris.

Stay Here: An antique-filled, three-bedroom Georgian home in the hip Shoreditch area of London ($377).