stiffness vs rigorousness

stiffness and rigorousness both are nouns.

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

As nouns, stiffness and rigorousness are synonyms defined as:

Other synonyms of stiffness include hardness, harshness, inclemency, rigor, rigour, rigourousness, severeness, severity.

stiffness (noun)	rigorousness (noun)
the physical property of being inflexible and hard to bend	something hard to endure
the property of moving with pain or difficulty	excessive sternness
the inelegance of someone stiff and unrelaxed (as by embarrassment)
excessive sternness
firm resoluteness in purpose or opinion or action