Um... I'm not sure if a white light would work well against a zombie. They don't feel pain, and thats what causes people to turn away from white light. As far as I know they mostly hunt through sound and smell.
One more vote for night sights. But thats only if my primary (AR-15), secondary (870P Max), thirdary (No.4 Mk1) and fourthary (vicious house cat) failed and I had to use my sidearm.