Dynamo,
You make a good point with the knife (pun intended), but I chose the lighter based on the scenario dreamed up by Backpacker Magazine: "Stranded in the mountains."
I take "stranded" to mean that I can't evacuate, which means that I must stay in place and await rescue. It also means that time is my enemy. Stressful, no?
So staying warm and hydrated while signaling for help would be my battle plan. With a lighter, I could start a signal fire asap--which will also keep me warm, give me light, and boil my water--instead of burning daylight trying to fashion a fire-making instrument from scratch with a knife, which could take a ham-fisted bonehead like me hours to do.