Dear CPF members;
I too have been long considering between these two lights. Some points I have recognized:
The Surefire L1 has two methods of turning it on (press switch or bezel twist, thus possibly more reliable) (the HDS has only the press switch).
The Surefire L1 does not have a light flicker effect on low, while the HDS is said to do so as a normalcy.
The Surefire length is much easier for handling the light and operating the tail switch with ones thumb, than it is with a much shorter light, as the HDS is (at least for my hand size/shape, and when wearing gloves, as I have tested with other similar sized lights).
The Surefire is more mechanically controlled rather than electronically controlled, which (I have read) is more reliable (but this remains to be seen overtime).
The Surefire L1 is a fourth generation light, the HDS is much younger and less tested in the field by time, thus the Surefire may be more dependable (again, this must be confirmed over time).
The Surefire L1 requires only one press or twist of the bezel for the desired light output. The HDS requires two or more clicks for any light level other than the primary or maximum. This can be burdensome if frequent short bursts of low light is needed.
Surefire is a larger company, thus potentially more stable as an insurance for the lifetime warranty.
But the HDS has its full regulation at all levels, much shorter length, two extra light levels, rechargable battery option, AA battery option (coming), and greater lumen output, unlike the Surefire L1.
A difficult decision.
Most sincerely;
FaithinGod