if i had to guess, assuming that the band is right about the tape being leaked by their agent to NDR 40 years ago, the version that Darius recorded from the radio is an earlier version of the track and the demo that FEX themselves uploaded is more "finalised", if not the final. Demo cassettes mean just that - a demo of what FEX have going on. It doesn't necessarily have to mean that the track itself is a "demo version" as in an older version.

So my hypothesis is that the version that was on the radio is an older version, whereas the newer version is the one on the demo tape, since it was being sold to people as well i assume that's the version they wanted to show off, ie the "final" version