Don't know if this has changed recently, haven't used Chromecast for a while now ?
Unless the rule has changed, just one thing to bear in mind with the Chromecast Audio that you need to be a paid Spotify customer to use the Spotify app.
Where the Chromecast Video dongle you can use both the Spotify Free version with the adverts (reduced streaming quality) or the Spotify paid version with non adverts and better sound streaming quality.
Just something to consider if you every decide to drop the Spotify subscription paid service to the free Spotify service in the future.
The Chromecast Audio works on a audio type connection to your sound device where the Chromecast Video works on an HDMI connection.