None of the above.

Headlights don't work well mounted low to the ground. The low angle creates lots of shadows due to surface irregularities. On a gravel road perhaps 1/3 of your light seems to disappear due to thousands of obstructions to the light path. Try your light low on forks off-road, then repeat route with light on handlebar - you'll see a big difference.

Lights mounted on the helmet have a another effect - they create little to no shadow due to the light source being practically in line with your eyes. The lack of shadowing leaves you less capable of resolving distance - your depth perception is diminished. This effect is most apparent on off-pavement routes, where you really need all your depth perception to dodge obstacles.

For most bicycling, lights work best mounted at or near handlebar height.