Your choice should be dictated by his assignment.
If he's a grunt, then a 123-powered light is appropriate, since he will have access to batteries. As a former grunt, I can tell you that I would given a lot to have a dependable, durable light like an L1 for putzing about at night when not in a tactical situation. The L2 is somewhat larger, and to a grunt, size/weight is EVERYTHING, believe me; even to the mainly mechanized grunts of today.
If your guy is a REMF, then it really doesn't matter. A small AA LED, such as the SL Junior Luxeon, light might be a better choice.
Mainly, whatever his MOS/assignment, he will probably appreciate something he can keep in his pocket a lot more than a larger "tactical" (God, how that term is mis-used /ubbthreads/images/graemlins/icon15.gif) light of some sort. At least, that's what I would have preferred back in the day.
And if is a grunt, DEFINITELY get the red and green filters for whatever you buy. The green is for use with NODs; causes less "flare" than other colors, especially white.