I subscribed Google Play Music when they first offered the service and enjoyed it but I feel Spotify is much more conducive to 3rd party add-ons, I couldn't find many differences with the subscription based libraries (the ability to upload to google play is nice...but also free) and Spotify actually has an API, Desktop Application, actual 3rd party support. In my experience any 3rd party support (last.fm, etc) in Google Music breaks every release, which was the main reason why I switched back to Spotify. It may be a little more $ each month but I found the advantages Spotify has over GPM were worth the cost.
