@mikado,
We're maintaining two parallel release branches at the moment while things settle. It's fair to say that the 1.5.x branch is the maintenance release and 1.6.x is the next mainline release (at least for AsciidoctorJ core). However, AsciidoctorJ is stuck waiting on Asciidoctor Ruby core to make that jump (and on me to get the AsciidoctorJ PDF repository fully setup).
My general advice is still the same. If you are just using AsciidoctorJ to convert without any customizations, then use AsciidoctorJ 1.5.5. If you're writing extensions or using the AST, then use AsciidoctorJ 1.6.0 alpha.
Cheers,