Bottom line: pick Barcelona for beaches, Gaudí and a coastal, walkable feel; pick Madrid for world-class art museums and nightlife. Barcelona has the Mediterranean, Gaudí's skyline (the Sagrada Família is arguably Europe's most extraordinary building — book timed tickets 2-3 weeks ahead in spring/summer, and Park Güell's monumental zone also needs advance booking) and the Gothic Quarter's maze-like lanes. Madrid counters with the Golden Triangle of Art — the Prado (Velázquez/Goya/Bosch), the Reina Sofía (Picasso's Guernica) and the Thyssen-Bornemisza — all within a 15-minute walk, plus better value and stronger nightlife. The two cities are linked by the 2.5-hour AVE high-speed train, cheaper than flying. If you have 8+ days, do both; first-timers with 4-5 days will find Madrid more concentrated and easier to plan.