Having worked in the lighting industry for the emergency vehicle industry, I will give you my 2 cents worth.

LED has no filament to break, as they are solid state.
HID consume large amounts of power and desepate large amounts of heat form their power consumption. HID takes time to come on, they are not instant like LED's

LED has a life expectancy of 50K hours HID are rated 2000-3000 hours and are not solid state. Plus the vibration of the road is hard on them, causing them to break down faster.
HID is much more to install as you have more components to hide and wire, including a large driver.

LED is the right way to go as far as light quality and brightness (kelvin). While HID's may be brighter, they are a whiter light, and can be hard to see at times from an approaching vehicle, unless you are right in direct line of sight.

Emergency vehicles mostly spec LED now, why? Visibility not only to see, but to be seen...Hence, LED is the way to go today.