With the accessory rail on most pistols a vestigial stub at best, having a small, compact unit capable of both aiming assistance and providing light is most efficient.
Streamlight's TLR-4 G Rail-Mounted Green Laser/Light Combination offers a Class 3R green laser and a 115 lumen light in a package designed to mount on full-size, compact and even sub-compact handguns. Obviously, it will work fine for longarms and shotguns as well, but in those cases, rail estate is not quite as limited as a pistol's accessory rail and streamlined units are less necessary.
With a run time of 1.25 hours with light and laser, 1.75 hours with light alone or 4 hours for laser alone, the single 3V CR2 lithium battery that powers the TLR-4 G should last quite some time. With the minimal real estate needed on the accessory rail and compact housing, the TLR-4 G doesn't add a lot of weight to your defensive sidearm, either—it's a featherlight 2.8 ounces.