Obvious the amount of light is important when you choose a headlamp. But don’t focus exclusively on the number of lumen, even more relevant is how the light is spread. Of course it must reach far but it must also reach down to your feet if you maybe are running. You need to have a uniform spread of light that is not very bright in one spot and too weak in another.
This light test was shot in August 2014 and includes some of the most popular headlamps.
LEDX Cobra 5500
LEDX Mamba 3500
LEDX Kaa 1500