unthoughtfulness vs thoughtlessness

unthoughtfulness and thoughtlessness both are nouns.

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

As nouns, unthoughtfulness and thoughtlessness are synonyms defined as:

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

unthoughtfulness (noun)	thoughtlessness (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