After discussing a bit with @Stefan via Kontalk voice messages about Kontalk and Signal, I my opinion is the following:
Signal is the better messenger, and while Kontalk goes more in the direction of security Signal goes more to usability which is more important to the average user.
BUT: Moxie Marlinspike, the founder of Signal, has very strange and (for me) unreasonable opinions regarding F-Droid and Open Source.
Now I came up with the following solution: Open Whisper Systems should employ @daniele_athome as main Signal developer instead of Moxie and everything would be fine! 