thoughtlessness vs unthoughtfulness

thoughtlessness and unthoughtfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
thoughtlessness	Yes	No	No	No
unthoughtfulness	Yes	No	No	No

As nouns, thoughtlessness and unthoughtfulness are synonyms defined as:

thoughtlessness and unthoughtfulness: the trait of not thinking carefully before acting

thoughtlessness (noun)	unthoughtfulness (noun)
the trait of not thinking carefully before acting	the trait of not thinking carefully before acting
the quality of failing to be considerate of others