Conclusion: pick Amsterdam for canals, masterpieces and a first-time visit; pick Rotterdam for architecture and a second Dutch trip. - Amsterdam: the UNESCO canal ring, the Rijksmuseum (Rembrandt, Vermeer) and Van Gogh Museum, plus world-famous nightlife — but pricier. - Rotterdam: a post-war modern architecture showcase (Cube Houses, Markthal, Erasmus Bridge) and Europe's largest port, with hotels €30–50/night cheaper and restaurants 10–20% cheaper. - Just 40 minutes apart by train (~€14 each way); Rotterdam works as a 5–6-hour day trip from Amsterdam. - With 4+ days, do both — a Rotterdam day trip captures 80% of the experience.