stiffness vs rigourousness

stiffness and rigourousness both are nouns.

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

As nouns, stiffness and rigourousness are synonyms defined as:

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

stiffness (noun)	rigourousness (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