The downroad 'reach' of headlamps is governed by US performance specifications (primarily FMVSS108 so in reality, the different 'advanced' light sources are designed to spread available light but are restricted in their reach by 'glare' requirements set by our government - until their regulations catch up with technological advancements, the down road reach of headlamps is pretty much the same regardless of light source. That said, the drivers eye often perceives a difference because in systems using HID, the greatly increased light availability is used more in side illumination and directly in front of the vehicle to give added 'comfort' value. This extra light near to vehicle can cause the driver to perceive he cannot see as far with HID because downroad illumination cannot exceed values set for halogen (currently) but the eye adjusts to compensate for the increase in light closer the vehicle - this is an on going battle for lighting manufacturers lobbying Washington to review their regulations to take advantage of the improving light source technologies.
In my view (for what it's worth) if you want more light for side spread and close to the vehicle (slow speed, off road driving) then HID and LED Headlamps are the way to go but if you're primarily driving highways at greater than 45mph, the HID and LED systems will not let you see any further down the road than traditional halogen sealed beams on low beam. The HID and LED systems do have the advantage that they put our a more natural 'daylight' color (appear bluish at night) which helps the driver see colors much better but again, the ability to see further down the road isn't really there.
Currently light sources can be categorized as HID giving most light, LED next, then Halogen and finally tungsten (some of the older sealed beam) but LED is expected to out perform HID in the next 5 years but bear in mind that it's the collection system (reflector + lens) that can make all the difference in how that light is put down the road.
Also, don't be tempted to use Projector lamps if you want performance as they may look cool (vehicle stylists seem to like them) but they actually collect much less light than the 7" round halogen units (when it comes to collecting light from a light source, you can't beat the "bigger is better" approach).
The big advantages of HID and LED systems is their power consumption, so if you want more light (ignoring down road reach) with less demand on the vehicle electrical system, then HID and LED have the edge (25w HID systems are roughly comparable to 55w halogen low beam light performance). There are higher output systems available but if they're marked 'off road use only' then they probably don't meet government regulations for highway use !
Just my 2 cents worth but hope it helps in making light source choices.
