Decide Between Two Tabletop Systems For A New Campaign

Two rulebooks sit on the table, both promising to be beginner friendly, and someone has to actually decide before session zero. This compares complexity, prep time, and how forgiving each system is of a table that improvises, then names one system rather than leaving you to compare page counts yourself.

Compare {SYSTEM_A} and {SYSTEM_B} for a new campaign with a group described as: [mostly new to tabletop roleplay, wants combat and story in roughly equal measure]. Cover rules complexity for a game master who is also learning, how much prep each system demands per session, and how forgiving each is of a table that improvises rather than following the rules strictly. Give one clear recommendation, stating the deciding factor.