Verdict first: Florence for Renaissance art, walkable history and romance; Milan for fashion, design and big-city nightlife. The high-speed train links them in about 1h40 with ~23 direct trains a day and advance fares from €18, so you can do both. Florence (~360,000 residents) is the first-timer's pick — the Uffizi, the Accademia's David and the Duomo are all walkable; allow 2–3 days. Milan (~1.4 million) is Italy's fashion and finance capital — the Duomo, The Last Supper and Galleria Vittorio Emanuele II fit into 1–2 days, but dining and shopping cost more; it does have 3 airports and the best onward links to Lake Como and the Alps.