Should you stay overnight in the Douro Valley?
Stay overnight if you want the Douro to feel like a place, not a pretty commute from Porto. Day-trip if wine is a curiosity; linger if it is part of the reason you came north.
Answer
An overnight changes the Douro from a scenic excursion into a slower travel day. You get the valley in softer light, the chance to eat without watching the return clock, and a better sense of why wine country is not just a backdrop for tasting notes. The trade-off is logistics. Without a car, you need to think carefully about how you move between train stations, towns and estates; with a car, tasting plans need restraint. For a tight Portugal route, a day trip is sensible. For a food-and-wine trip, the overnight is often where the Douro starts earning its reputation.