In a light system, bar-mounted lights should provide the majority of the lumens, flooding a wide beam down the trail. Mounted below your eye line, this light creates shadows helping you to spot rollers, rocks, and roots as well as creating some depth perception.
Helmet-mounted lights shine where you're looking, helping you see around corners, over rocks and anywhere your bars aren't pointing. This light doesn't need to have the power of the sun and should have a tighter beam pattern than your bar lights.
If possible the best option is to have both helmet and handlebar-mounted lights simultaneously to get the advantages of both setups to illuminate as much of the trail as possible.