Replaced the halogen with a HID. THE LED is the whiter of the two but that is because I chose a 5000k HID bulb as opposed to the 6300k my brother has, which is the same as the LED I have fitted. The HID has a far greater beam strength, no question about it.
Other observations:
Beam pattern - although I can't road test I can see from the way the light fills the reflector that the HID is the better of the two options. Whether the LED is acceptable or not I can't say but to look at it is very eye catching. If you ride at night the beam throw is more important.
Heat - both types of light unit produce it. With the HID the lens gets hot, just as with a halogen. The LED leaves the lens completely one-hot (!). Instead the fan unit at the rear of the LED gets hot. You can hold onto it but it is hot. With the HID the ballast unit also gets just as hot. You can't produce this level of light and not get the heat.
Installation - electrically the LED is plug and play. If you want to fit the rubber boot you will need to modify it to fit over the fan unit, but other than that you just fit them and off you pop. The HID's are a different kettle of piranha's. Mechanically these are very easy to fit in the light unit, but it's fitting in all the ancillary bits (ballasts, chokes, control relay etc) that is a headache. I've fitted a set before and to do an half-way decent job is time consuming as you have to chop and change the wiring harness that comes with the kit. Neither type of light system requires any mods to the bike's wiring, so to reverse the installation is easy.
Any questions?