Keep in mind, when comparing bike lights, a well matched, focusing, lens and reflector can work wonders for any bulb. For the time being I'll stick to watts with the following emotional brightness scale to SEE by. Useless in the rain. To been SEEN is a combination of many things including brightness of light. The brighter the light the further away you'll be seen by road users entering and crossing your line of travel.
Batteries can be a complicated business.
There is so much to know about batteries your buying decision could be paralysed by information overload. Bicycle lighting is a highly competitive market and manufactures, honest as they wish to be, are forced to present their products in the best possible light. Manufacturers will indicate how long the light should last between charges. These times are quite often optimistic 'theoretical' best case scenarios which don't mention your lights may be brown in the last 15 minutes of use or, performance as the battery ages. You don't usually find that stuff out until you read the rest of the owners manual.