Generally the higher the wattage the less run time. LED's don't use much watts (power), but aren't all that bright - yet. At least for biking purposes. But you need 15 to 20 "watts", as they are sold, to get a good line of sight on single track. HID's are bright and tend to use less battery power, or give you longer burn time on your battery.
So, to simplify it as it stands, you'll probably end up spending between $190/$220 for a light that will give you both battery run time of about 2+ hours and shine a far enough path so you're not riding at 6mph. Good to have a backup light, even of lesser light in case something happens. Serfas has a bright LED light with head strap for about $30 that you can mount on your helmet or wrap on your handlebars. It uses 3 AAA batteries and runs almost as bright as a $190 light. Almost. Good safety light if your battery goes out or light gets damaged from wayward trees.
To justify the purchase it won't be too long before it gets dark early. :-)