second-place finish vs runner-up finish

second-place finish and runner-up finish both are nouns.

Word	Noun	Adjective	Verb	Adverb
second-place finish	Yes	No	No	No
runner-up finish	Yes	No	No	No

As nouns, second-place finish and runner-up finish are synonyms defined as:

second-place finish and runner-up finish: a finish in second place (as in a race)

second-place finish (noun)	runner-up finish (noun)
a finish in second place (as in a race)	a finish in second place (as in a race)