Nest Secure: The Google Assistant is built in. You can control your home security system, just say, "Ok Google, set security to away and guarding." It also can control compatible connected-home devices, manage tasks, and get answers powered by Google Search. (Mobile Notifications require working internet and Wi-Fi). Nest Secure is not compatible with Amazon Alexa.
Smart Security Systems: Smart Security uses “skills” provided by Alarm.com to control your system and compatible devices through Alexa, Siri, and the Google Assistant. For example, you can say “Alexa, ask Alarm.com to arm the security system.”