marriage proposal vs proposal of marriage

marriage proposal and proposal of marriage both are nouns.

Word	Noun	Adjective	Verb	Adverb
marriage proposal	Yes	No	No	No
proposal of marriage	Yes	No	No	No

As nouns, marriage proposal and proposal of marriage are synonyms defined as:

Other synonyms of marriage proposal include marriage offer, proposal.

marriage proposal (noun)	proposal of marriage (noun)
an offer of marriage	an offer of marriage