Finally I switched solely to Signal Private Messenger today. Signal has an Android and iPhone app and even a Desktop version (for Linux, Mac and Windows) and they work well. They also feature voice and video chat, so basically I'm not missing any features. It is also Open Source and even Bits Of Freedom is positive about it.
It took me quite some time to phase out Whatsapp, since a lot of friends of mine are (still) using it. This phenomenon is called "vendor lock-in". Now I will that same issue with Signal, since the systems are not interoperable, but at least I'm now using a secure and privacy aware system. The company behind Signal ( Open Whisper Systems ) has an extremely good track record of practicing what you preach, so my data (and privacy) are not abused or misused.
Since Whatsapp got acquired by Facebook, they made quite some "evil" decisions, such as mining (and aggregating) your Whatsapp and your Facebook data. Since they are a company who wants to make a profit using your data, it was to be expected. Facebook (and thus Whatsapp) make money on a per user basis, by using and selling your personal data (or a derivative thereof). So I made the concious decision not be a part of that ecosystem anymore. The first step now, was migrating away from Whatsapp. More will follow.