The short answer
Stardew Valley is the farm-centred choice; Terraria puts building, exploration and combat at the centre. Our recommendation is to choose the activity you want to repeat, rather than treating visual similarities as evidence that the games are interchangeable.
When to choose Stardew Valley
Choose a farming routine if you like returning to a familiar place and deciding what to work on next. Before buying, ask yourself whether maintaining that routine sounds satisfying. If it sounds like an obligation, do not let the relaxing-game label make the decision for you.
When to choose Terraria
Choose a sandbox adventure if finding a direction, investigating a world and building around discoveries sounds appealing. Our trade-off is that self-directed play can leave a new player unsure what to do. Someone who wants a clearly signposted story should consider that before choosing.
Choosing with friends
Both Steam listings offer cooperative play. Agree on the shared activity before asking which game is better: improving a home base and routine, or setting out to explore and fight. Then check the current store features for the way your group plans to connect. This comparison does not claim cross-platform compatibility.
Use a two-question shortlist
First ask what you want to do during an ordinary session, not only the most exciting moment in a trailer. Then ask what kind of friction bothers you more: managing a routine or deciding your own direction. If the answers split your group, consider alternating games instead of trying to force a single winner.