Should I get a combination smoke + CO detector or separate units?

Reviewers are split. Verified-purchase Costco buyers and Good Housekeeping's writeup of the Kidde Ring combo argue that one device on the ceiling is simpler and cheaper to maintain. Several r/firealarms and r/smarthome threads push back, noting that smoke and CO sensors have different ideal mounting heights and different replacement schedules, and that a failed combo unit takes out both protections at once.