Spain's two biggest cities have a genuine rivalry, and travelers deciding between them are really choosing between two different identities — Barcelona's Catalan coastal culture versus Madrid's role as the country's political and artistic capital.
Barcelona's skyline is defined by Gaudí — the Sagrada Família, Park Güell, and Casa Batlló are unlike anything else in Europe. Madrid's landmarks — the Royal Palace of Madrid, Plaza Mayor, Puerta del Sol — are grander and more classically Spanish, reflecting its history as the seat of the monarchy.
Madrid has the stronger art case: the Museo del Prado, Museo Reina Sofía, and Thyssen-Bornemisza Museum form the so-called Golden Triangle of Art, one of the best museum clusters in the world. Barcelona counters with the Museu Picasso and a much stronger case for architecture-as-art via Gaudí's buildings themselves.
This is the clearest difference: Barcelona has a proper city beach at Barceloneta, letting you swim and sightsee in the same day. Madrid is landlocked in the center of the country — no beach, but a slightly cooler climate in summer and a strong claim to being more "authentically Spanish" for travelers who want fewer international crowds.
Both cities take tapas seriously, but differently — Madrid's Tapas & Wine Tasting Tour culture and Flamenco Show scene run late into the night, while Barcelona's La Boqueria Market and Catalan cuisine lean more toward seafood and Mediterranean influences. Nightlife is later and more intense in Madrid; Barcelona's is spread between the Gothic Quarter and the beach clubs.
Choose Barcelona for Gaudí's architecture, a beach, and a more relaxed Mediterranean pace. Choose Madrid for world-class art museums, grander royal architecture, and Spain's most intense tapas and nightlife scene. Both are well connected by high-speed rail (about 2.5 hours apart), making a combined trip realistic if you have a week or more.