You don't even have to specify the speaker: By making it the default audio projector, just use the Home-standard "Hey Google" prompt and voice-command away. (You'll still have to talk into whatever device has the Home app installed, not the speaker in question.) Users can even add a Bluetooth speaker to a group, if they have one set up. This effectively enables multi-room audio without needing a Home device or Chromecast in each space.
Google Home supports Google Play Music, Spotify, Pandora and YouTube Red, according to its support page. By comparison, Amazon's Alexa is compatible with Amazon Music, Spotify, Pandora, iHeartRadio, Deezer, TuneIn, SiriusXM and GimmeRadio.
Update 4/2/18 4:10PM ET: This post has been updated to clarify which services Google Home is currently compatible with.