Think about the light first
A bright desk, a street-facing sitting room and a bedroom each need something different from a blind. Decide whether you want to filter daylight, adjust its direction or reduce it as much as possible. Privacy also changes when lights come on inside, so consider the window during the evening as well as the day.
A Roman adds a generous area of decorative fabric. A roller keeps the outline simple. Venetians and verticals offer adjustable slats or vanes. The best choice comes from matching the way the blind works to your daily routine.