Sooner or later, in your decision process, you will have to determine whether you need to SEE or just be SEEN by the light on you bike. Will you have to illuminate the street to avoid all the dangers we know too well or are simply advising other road users of your existence. The brighter your light, the further you can see and the more likely a motorist will see you. Brightness has a price though. Sometimes a very big price.
Bright lights consume a lot of battery power.
Defining brightness is not such an easy task if, as is the industry wide practice, we avoid units of measure that convey actual brightness. The bike light industry is fixated on 'watts' to convey brightness. This measure of power consumption is an adequate but not precise measure of luminosity. It is better understood than 'lumen', 'lux', 'candela' or 'candlepower'. At least 'watts' are useful when trying to approximate how long your light will be bright.