Definitely a knife. I carry a box cutter around with me at work all the time. I just cut open more boxes than I look for things in dark / unlit places.

Do I have my phone? If so, knife. While my phone’s flashlight isn’t as good as a real one, it’s good enough for most of the times I need one.

Am I cheating?

I’d prefer a knife. Both, of course, are in my EDC, but I’d choose a knife over flashlight. It’s the tool I’m most comfortable with.