I find Jem to be pensive and Scout to be impulsive.
Jem is a thinker, he processes things. Scout notes this as she refers to his need for alone time, she often thinks he must be thinking about their mother who is no longer with them (she died). He also processes the trial and finds it unfair. Well, so is life, but Jem can't justify this very well in his brain. Jem also takes time to consider Dill's dare to touch the Radley's house. It took him 3 days.
Scout, on the other hand, acts upon her ideas instead of reflecting on them when they come in her head. She stands up to Walter Cunningham Sr. in the mob, she gets in fights, and she speaks on behalf of the class to Miss Caroline.