Sibiu is itself a Transylvanian city, so this isn't a comparison of unrelated places — it's a choice between touring the whole region's castles and citadels or settling into one exceptionally well-preserved historic town as a base. Here's how the two experiences actually differ.
A regionwide Transylvania trip puts Bran Castle, Peleș Castle, and Corvin Castle (Hunedoara Castle) all within reach on one multi-day loop. Based in Sibiu, you can still reach Corvin Castle (Hunedoara) as a day trip, but Bran and Peleș sit too far north to fit into a single day from there.
Transylvania lets you compare Council Square (Piața Sfatului, Brașov) against Large Square (Piața Mare, Sibiu) and the wider Sibiu Historic Center in one trip. Staying in Sibiu means lingering on that same square, Piața Mare (Great Square), plus the Council Tower (Turnul Sfatului) and the Bridge of Lies (Podul Minciunilor) beside it.
Transylvania's cultural stops span the Black Church (Biserica Neagră, Brașov) and the ASTRA National Museum Complex over in Sibiu. Sibiu itself goes deeper, adding the Brukenthal National Museum and the Lutheran Evangelical Cathedral of St. Mary alongside its own ASTRA National Museum Complex (Open-Air Museum) — enough to fill two or three unhurried days.
Transylvania itineraries fold in Alba Iulia Fortress and the Sighișoara Medieval Citadel as stops along the route. From a Sibiu base, the standout excursion is the Transfăgărășan Highway & Bâlea Lake drive, with Sighișoara Medieval Citadel still reachable as a day trip and the winter Christmas Market (Winter) drawing visitors who stay put.
Choose Transylvania for a multi-city loop through castles, fortresses, and medieval citadels spread across the region. Choose Sibiu for a single well-preserved base with strong museums, a dramatic mountain day trip, and everything walkable. Most travelers base in Sibiu and use it as a launchpad into the rest of Transylvania.