An HID bulb has a much larger light output source than a halogen bulb. So an HID bulb in a halogen housing will send light where it wasn't intended (usually into the eyes of oncoming traffic).
A well-built LED chip perfectly mimics a halogen bulb, and offer considerably more light where you want it, and nothing where you don't.
Because of the higher color temperature of LEDs, they generally don't seem to cast as much light down road. So I left the high beams as halogens, but I put some decent LEDs in place of the low beams in our 04. These have a smaller light output point than the halogen, and as a result, a slightly narrower beam.