I'm lukewarm on the light on the handgun, for the reason ET gave.
There's so many variables that can affect this issue, that you have to keep in mind. Cops have to go catch azzhats in buildings; Hopefully they brought friends and a dog when they have to go on offense.
An armed citizen most likely will be investigating a noise. In those cases things are extremely fluid. You point the light the wrong direction and turn it on, and you're silhouetted and an easy target. Point it a different direction two seconds later and the bad guy is blinded, throws down his gun/knife and gives up. And what you can do depends on state laws, too, and the exact situation.
Besides that, if you have the rail empty on the pistol, you can mount all sorts of cool things, like a bayonet, a flare gun, or a mini-crossbow...
