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What are the advantages of each setup. Obviously more is better :) but if you had to choose one, which would it be and why? The scene would be in a tight wooded single track.
Bar light reverses the above two points:
If you are riding MTB then using both is highly suggested. If you can only aford one then go with the helmet mounted light.
Helmet light is my preference. The light is aimed to where you are looking - though this means that the lamp must be positioned correctly or you get a crook neck. On technical single track, corners can be a problem with handle bar mounted lights as you cannot see around until you are turning - often too late. Helmet light also provide a better angle down to ground level, casting fewer shadows in hollows and over dips. Mounting on handle bars creates shadows can show up small bumps better, but the depth of the hole on the otehr side is hidden. I prefer to go over small stuff I did not see to big stuff I mistook for small stuff.
Weight with a decent lamp and battery pack can be an issue - a separate battery pack is a better option than helmet mounted batteries, and some people have questioned what happens to the helmet and your head in a prang when you have a lamp strapped to it.
Handle bar lights are better in rain and snow, where the light from a helmet reflects straight back into your eyes.
However, with modern LED lamps competing with the sun for total light output, it makes less of a difference than 15 years ago when we rode on tight technical track on 5W or 10W halogens - Night riding was largely by feel and memory backed up by quick reactions in those days.