Well, the girl in red will ask you to either kill a number of a specific monster in a specific area or to kill the wise man.
The demon only asks you to kill to girl.
The drops are about the same. However, I feel like the girl shows up a bit more often than the demon, so you might get more chances for rewards by helping the girl. The trade off is you'd probably be killing the wise man once every 2-3 times he shows up and you'd always have to kill the demon.
I personally chose to help the demon.
