HID's are really only worth it if you have projector lenses
What diff do they make ?
And most cars with projectors are Hid's aren't they ?
HID's emit light in a different pattern to to filament bulbs, mainly due to the nature of their construction. Projector lenses tend to be able to correct the beam better, so as to render the pattern the same, irrespective of whether the bulb is HID or filament.
Not all cars have them though. For instance, the ones that I drive:
Clio 2000 - Projector - Filament bulb
Mercedes 2002 - Normal - HID
Disco3 2005 - Projector - HID
Disco3 2008 - Normal - Filament