grimness vs rigorousness

grimness and rigorousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
grimness	Yes	No	No	No
rigorousness	Yes	No	No	No

As nouns, grimness and rigorousness are synonyms defined as:

Other synonyms of grimness include asperity, hardship, rigor, rigour, rigourousness, severeness, severity.

grimness (noun)	rigorousness (noun)
something hard to endure	something hard to endure
the quality of being ghastly	excessive sternness