Lights work. Unmounted lights have their advantages, as said, they can point where the muzzle isn't, and not give away your center of mass.
Weapon lights are a bagdad fad from spec ops/MP use. Most civilians don't or won't learn how to really use them, nor will they do any better than a hand held. Please be reminded most weapons mounted light users are working as a team with 2 to 9 others. Alone, things can get very complicated trying to control a situation if the BG's buddy steps out of a hallway.
I would definitely recommend a light. LED's above 60 lumens, with a low level 5-10 lumen secondary, will help navigate and illuminate better overall. Be careful about buying lights that work off of 123 lithium's from a cost standpoint. They can be cheap if bought in bulk, but you're almost forced to. Rechargeable AA's are a better deal, work in lots of other stuff, and don't cost you an arm and a leg if you need more at the 'Mart. You can always just stick in alkalines. There a bit smaller and take up less space, too.
If there is an incident, the weapons mounted light may go with the firearm when the police need it for evidence . . .