weightiness vs heaviness

weightiness and heaviness both are nouns.

Word	Noun	Adjective	Verb	Adverb
weightiness	Yes	No	No	No
heaviness	Yes	No	No	No

As nouns, weightiness and heaviness are synonyms defined as:

weightiness and heaviness: the property of being comparatively great in weight

weightiness (noun)	heaviness (noun)
the property of being comparatively great in weight	the property of being comparatively great in weight
the relative importance granted to something	used of a line or mark
	unwelcome burdensome difficulty
	an oppressive quality that is laborious and solemn and lacks grace or fluency
	persisting sadness