Blue Pure 311i+ Max vs C909

Air quality sensor is a yes/no feature — a product either ships with it or it doesn't. There is no middle ground and no firmware update will retrofit it. Boolean specs tend to fall into two buckets: nice-to-haves that occasionally swing a buying decision (think wireless charging, biometric sensors, headphone jacks) and hard requirements that rule a product out entirely (think water resistance for swimmers, expandable storage for media collectors). Decide upfront which bucket this feature falls into for your use case. If it's a nice-to-have, treat its presence as a tiebreaker between otherwise-equivalent options. If it's a hard requirement, filter it first and only compare among products that include it — there is no point ranking a product that fails your gate.