I did some more research on this, and it seems that both terms were used. Already in 1920, Alfred Hoche (a psychiatrist) and Karl Binding (a jurist) published a paper called "Die Freigabe der Vernichtung lebensunwerten Lebens", where they discuss the economical benefit of killing treminally ill or severely mentally disabled people. A lot of the terminology of that paper made its way into the Nazi vocabulary. Later both "lebensunwertes Leben" and "unwertes Leben" were used to describe people that in some sense or other did not conform to the desired norm, that is, again handicapped (mentally or otherwise) people, Jews, Sinti, Roma, homosexuals... The idea of enforcing strengthening the population by erasing the influence of "weaker" elements was a popular idea at the beginning of the last century, not only in Germany, and today reads like a horror story.