It’s available now or at least I’m seeing the option to set it up via the app.
Yup. I was using it last week. I am in the beta program, but not the Google one, so assume open to everyone. Switching between the two is a bit involved, so you will probably be sticking with just the one, although if you have 2 paired I guess you could have 1 set to Alexa and the other set to Google.
Personally I prefer Alexa as 1) better wake word, and 2) you can set her to not acknowledge after as request.
For example:
Me: Alexa, turn on the light.
Alexa: Quiet tone.
Me: Hey Google! (shudder), turn on the light.
Google: Turning on the light!