Which for caramel popcorn. as we dont make junk food i dont know. sorry if it is a basic question.
Most of the recipes that I've seen just call for brown sugar.
The difference between to two sugars is that dark brown has more molasses in it. I would use the dark just because it would be darker in color.
I know this isn't what you asked, but this is just a tip...I hardly ever buy brown sugar anymore. I keep a bottle of molasses on hand and just add a little to regular sugar for recipes.
For caramel corn I use which ever I have the most of....as my MIL (God rest her soul) would say...."couldn't tell the difference from the back of a galloping horse."
At some point, years ago, I remember reading that unless a recipe specifies dark brown sugar, you should always use light brown.
But....I buy a medium brown sugar from a restaurant supply. Problem solved, I use it for everything.
I'm another who makes their own brown sugars depending on need. We eat from a food storage program and, for ease, I store white sugar & molasses. I make up a pound at a time.
Use what you have and what you like. The difference is more molasses in dark brown and some like that better, while some don't. I used to prefer light but gettin' older I like molasses more so I like the darker stuff now. I figure I could always lighten up the dark stuff by "diluting" it with sugar if something actually required a lighter brown sugar. Or make your own as suggested and make it the color you like.