According to this article, which was written in 2014, WhatsApp had partnered with Open Whisper System to make its (text based) messages encrypted. So what's the big change now that Whatsapp's announcement that it now uses encryption keys?
In what ways, if any, is using Signal Private Messenger more secure than using WhatsApp?
E2E encryption was supported by the Android client but not the iOS client until now. For this reason there was an feature to disable the encryption by the server side. This rendered the E2E encryption useless. Now the protocol is fully implemented for all clients so that tampering of the encryption by the server side should not work anymore. However, WhatsApp could change the client code at any time to implement a backdoor or something.
In what ways, if any, is using Signal Private Messenger more secure than using WhatsApp?
Beside that WhatsApp could change any code at any time for any reason and nobody would notice it immediately there's still the problem with meta data. The E2E encryption is only applied to the content of the conversation. Everything else is just protected by transport security. WhatsApp also has access to the contact list of all users by default. The biggest problem is that WhatsApp belongs to Facebook and the business model of Facebook is analyzing user data so you can not assume that your privacy really matters to them. However I must admit that the business model of Open Whisper Systems isn't very obvious either.
Summary
Things WhatsApp can access at the moment:
Things which are protected:
Some people say that the introduction of the axolotl protocol was just to get users back from Signal, Threema, Telegram and similar services.
