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Pick Oberhofen for the setting by the water: the little lakeside tower, the grounds and the rooms inside make an afternoon of it. Thun is more about the medieval keep above the old town, its history displays and the climb towards the towers. Calling them both castles hides how different they feel.
One is enough if you want a relaxed day. Check each museum's opening calendar, especially outside summer; park access and museum entry have separate arrangements. In Thun, expect stairs inside the keep. At Oberhofen, leave time for the lakeside grounds instead of rushing back after the photograph.