Activated carbon vs HEPA — which should I buy?

They solve different problems. Activated carbon (EPAuto, Spearhead, Puroma in this roundup) targets odors, VOCs and smog smells while preserving airflow. HEPA (the Bosch C-series) targets fine particulates, pollen and smoke at much higher efficiency but restricts airflow. Reviewers who live in wildfire zones or have allergies tend to choose HEPA; reviewers chasing fresh-smelling cabin air with no AC penalty tend to choose carbon.