plainness vs simpleness

plainness and simpleness both are nouns.

Word	Noun	Adjective	Verb	Adverb
plainness	Yes	No	No	No
simpleness	Yes	No	No	No

As nouns, simpleness is a hyponym of plainness; that is, simpleness is a word with a more specific, narrower meaning than plainness:

Other hyponyms of plainness include chasteness, restraint, simplicity, austereness, severeness, severity, bareness, starkness.

plainness (noun)	simpleness (noun)
the appearance of being plain and unpretentious	the quality of being simple or uncompounded
an appearance that is not attractive or beautiful	lack of ornamentation
clarity as a consequence of being perspicuous	freedom from difficulty or hardship or effort
the state of being unmixed with other material	a lack of penetration or subtlety