Adult. Even a smart kid isn't taken seriously. I like being proactive & responsible. There are way too many adults who act like kids. Flipside, some kids act more mature than adults!
Well, being a child you have less responsibility, but being an adult you get to do more things. I think it's a draw, or just depends on your preference.
I like having my independence as an adult but I liked not having to pay bills when I was a kid. It was nice to be a kid and not have any worries at all. I think I would go with being an adult in the end though.