Idealy, you want TWO lights, because scanning with a gun mounted light is heavy, cumbersome, difficult, less effective, and unsafe. One light should be handheld (or mounted to a hat) and used to scan quickly back and forth continuously to spot the eye reflection of predators approaching. With a handheld light you can very easily swing the light 270 or 360 degrees with minimal head/arm movement. Try doing that with a light mounted on your 8 pound rifle for a 20 minute stand and get back to me on how stupid an idea that was. You'll be able to spot eyes 300-500+ yards away with a decent light. As the predator approaches to around 100-150 yards you may be able it ID with your scan light, maybe less, maybe more.
Somewhere in there you switch over to your gun mounted light, which is typically the brighter of the two lights, and use it to positively ID and shoot.
If you only plan on buying ONE light, or if your area prohibits gun mounted lights, then just use a handheld light to both scan and shoot. It isn't terribly hard to hold the light against the rifle forearm as you shoot. That's what I do, works fine.