Choosing bicycle lights starts with the route you ride. On illuminated city streets, front and rear lamps mainly help other road users notice you in time. An unlit road asks more of the front light: it must also reveal potholes, bends and the edge of the road. A brightness figure alone cannot tell you whether it will do that. Look at how evenly the beam covers the ground and whether it gives you enough view ahead at your usual riding speed.
Aim the front lamp at the road rather than into the eyes of people coming toward you. A powerful light pointed too high can make it harder for drivers and cyclists alike to see. Give the rear lamp an unobstructed position; a bag, long jacket or mudguard should not hide it. The mount matters too. If a lamp slips or twists on rough pavement, even a well-shaped beam soon points in the wrong direction.
Before buying, compare more than the advertised runtime. Think about how easily you can recharge the lamp or change its batteries. Cold weather and a brighter setting can reduce the time available, so a backup light is useful on a long evening ride. A quick-release mount makes charging easier and lets you take the lamp with you when parking. The right set lights your actual route, stays visible from behind and lasts until you get home.