When we talk about lights, lumen count usually grabs the headlines, but the beam shape is equally, if not more important. Ideally, you want something that will have a smooth transition from flood to spotlight with no dark spots or hotspots as this can distract from the trail.
If you're doing lots of high-speed riding and your trails aren't all that curvy, a narrow spotlight beam will suit, but if your local singletrack is twisty and more technical, look for something wide and bright.