I've read the documentation https://developer.android.com/google/gcm/index.html
and the only difference I can see is that a CCS (XMPP) based server allows bi-directional communication directly between an android device and the GCM server.
Are there any other reasons why you would choose CCS (XMPP) over HTTP or vice versa?
XMPP messaging differs from HTTP messaging in the following ways:
Upstream/Downstream messages
Messaging (synchronous or asynchronous)
JSON
Plain Text
Multicast downstream send to multiple registration tokens.
You can broadcast a message to 1000 devices at a time with a single http call to gcm. For broadcasting http is better than CCS.
From Google docs :
You can continue to use the HTTP request mechanism to send messages to GCM servers, side-by-side with CCS which uses XMPP. Some of the benefits of CCS include:
The asynchronous nature of XMPP allows you to send more messages with fewer resources.
Communication is bidirectional—not only can your server send messages to the device, but the device can send messages back to your server.
The device can send messages back using the same connection used for receiving, thereby improving battery life.
CCS (XMPP) is asynchronous, which means it should be faster than HTTP. It also uses the existing GCM connection on the device to send messages from your app to your server (which saves battery, since you don't have to open your own connection to your server).
On the other hand, HTTP is much simpler to code, so unless you need the bi-directional functionality or you need to send messages in a very high speed, I'd stick with HTTP.
unfortunately google cloud platform will disable the XMPP API after a year https://cloud.google.com/appengine/docs/deprecations/xmpp
I'd choose XMPP to save devices battery cause its one of the big concern nowadays!