overnight case vs overnighter

overnight case and overnighter both are nouns.

Word	Noun	Adjective	Verb	Adverb
overnight case	Yes	No	No	No
overnighter	Yes	No	No	No

As nouns, overnight case and overnighter are synonyms defined as:

overnight case and overnighter: a small traveling bag to carry clothing and accessories for staying overnight

Other synonyms of overnight case include overnight bag.

overnight case (noun)	overnighter (noun)
a small traveling bag to carry clothing and accessories for staying overnight	a small traveling bag to carry clothing and accessories for staying overnight
	a guest who stays overnight