https://www.cnet.com/news/simplehuman-smart-mirrors-showcase-sound-google-assistant-ces-2019/

You can control both models through the Simplehuman app, which also adds features like setting alarms and expands control over sound, lighting and sensor activity.

The difference between the Hi-Fi and Hi-Fi Assist is one very big thing: Google Assistant. The Sensor Mirror Hi-Fi Assist works with Google Assistant so you can control audio functions via voice commands. You can also ask Google for things like the weather or information about your commute.