Turku and Rovaniemi represent two entirely different versions of Finland: one is the country's oldest city, a medieval river town in the southwest, the other is the official Arctic Circle gateway built around snow, reindeer, and the northern lights. The right pick depends entirely on what kind of trip you want.
Turku is Finland's former capital, and it looks it: Turku Castle and Turku Cathedral anchor a genuinely old European city with centuries of layered history. Rovaniemi was largely rebuilt after WWII and reads as a modern, purpose-built base for Arctic tourism, centered on the Arctic Circle Marker & Monument rather than any old town.
Turku's cultural draw is Aboa Vetus & Ars Nova, a museum built over excavated medieval ruins, alongside the Cathedral Church of Turku and the atmospheric Old Great Square. Rovaniemi's equivalent is the Arktikum Science Centre, which explains Arctic life and the science behind the aurora rather than centuries of urban history.
Rovaniemi is built around bucket-list Arctic experiences: Santa Claus Village, Reindeer Sleigh Rides, Husky Sledding Adventure, and nights watching for auroras from Glass Igloo Hotels. Turku's experiences are gentler, built around Archipelago Cruise and River Cruises through the Market Square waterfront.
From Turku, Naantali and the summer greenery of Ruissalo Island make for easy, low-key escapes into the archipelago. Rovaniemi's nature is far more extreme: Snowmobile Safari Tours, Ruka Ski Resort, about 200 km away, Northern Lights Aurora Tours, and a soak at the Hotel Aakenus’s sauna after a day in sub-zero cold.
Choose Turku for medieval architecture, archipelago cruises, and a relaxed southern-Finland city break. Choose Rovaniemi for Santa Claus, reindeer, huskies, and a real shot at the northern lights. Winter travelers should lean Rovaniemi; summer travelers should lean Turku.