OS statcounter seems to get to similar numbers, with significant regional differences. Which data source would be more repräsentative and reliable ?

Debian is not as easy as Ubuntu to set up, but much easier than in the past. And once set up, it’s rock solid and has long term security updates, so if one can get a little help with the installation, I’d recommend it even to novice users.

And Arch is fantastic if somebody wants to learn more about Linux, because of its fantastic wiki.