title: When should I use GCM device groups versus topics?
body: <p>I am looking into incorporating Google Cloud Messaging into my app.  I would like to send messages to all devices associated with a user.  At first, it looked like GCM's <a href="https://developers.google.com/cloud-messaging/notifications" rel="noreferrer">device group messaging</a> would be the way to go.  But that requires managing registration IDs and notification keys on my server.  It seems like it'd be easier to use GCM's <a href="https://developers.google.com/cloud-messaging/topic-messaging" rel="noreferrer">topic messaging</a> where the topic is keyed on some shared user information like a user ID.</p>
  
  <p>I intend to use GCM to send data to both Android and iOS apps.  I will be sending both background "content available" (to use the Apple Push Notification terminology) messages and user-visible notifications.</p>
  
  <p>What advantages are there, if any, of device group messaging over topic messaging?</p>