A helmet-mounted light will move with your head and always point where you are looking. On trails with lots of technical features and tight turns, a helmet-mounted light is invaluable.
Meanwhile, a handlebar-mounted light will point toward where your bike is headed — which is not always the direction you want to go.
In low-light and nighttime conditions, we recommend riding with both a helmet light and a handlebar light. If you must choose one, a helmet light is a top priority.