Something to think about with a light, that most people don't, is how the beam is shaped.
Lights can have throw (they'll send a focused beam a long way) or spill (they flood a wide area with light). If you're planning on being outside a lot with a rifle, throw would be more important. But if you're using it indoors I'd prefer more spill.
Unless it's for a rifle that'll be used primarily at beyond 75 yards, I prefer a light with more spill than the Surefire X300U / M600U style lights.
