simpleness vs plainness

simpleness and plainness both are nouns.

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

As nouns, plainness is a hypernym of simpleness; that is, plainness is a word with a broader meaning than simpleness:

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