Brightness can be measured in a number of ways, but the most common unit of brightness you’ll encounter are lumens. Lumens represent the power of the light emitted.
As a general rule, as you go up in price you get more lumens output. Lumens aren’t the most accurate measurement of brightness, as it doesn’t take into account how wide or narrow the light beam is and therefore how that brightness is dispersed over an area. It also isn’t standardised across brands, so use it as a guide. Lux is a better indication but used more rarely.
As a rough guide, around 300 lumens will light up the trail adequately, and a second light with about 150 lumens would work for your helmet light. The high-end lights are now putting out over 2,000 lumens, as a guide.
Some riders prefer to have the brighter light on their helmet, but this is a matter of personal preference. See what works better for you.