Do I really need a 'smart' CO detector, or is a regular plug-in fine?

For pure life-safety, any UL 2034-listed alarm is adequate, and specialist communities like r/firealarms and r/AirQuality routinely point shoppers at the cheaper plug-in Kidde and First Alert models. The case for a smart unit is remote notification when you're away from home, integration with a Ring, SmartThings or Matter hub, and a digital readout of ambient CO ppm. If you have a gas furnace, attached garage, generator or rental property you don't live in, the app-alert feature is the main reason to pay the premium.