The only feature which makes me want to think about Pubnub instead of Ably is GCM integration. Pubnub offers really simple GCM integration where you can publish a message to through the Channel as well as GCM. This is really important for Applications where messages cannot be lost if the user is offline.
It's THE feature which can make Ably the obvious first Choice.
The comparison table between Ably and Pubnub and Pusher anyway makes Ably the frontrunner. But GCM, it will be really really helpful.
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I am glad to read it.