toss-up vs even chance

toss-up and even chance both are nouns.

Word	Noun	Adjective	Verb	Adverb
toss-up	Yes	No	No	No
even chance	Yes	No	No	No

As nouns, toss-up and even chance are synonyms defined as:

Other synonyms of toss-up include tossup.

toss-up (noun)	even chance (noun)
an unpredictable phenomenon	an unpredictable phenomenon