upper class vs upper crust

upper class and upper crust both are nouns.

Word	Noun	Adjective	Verb	Adverb
upper class	Yes	No	No	No
upper crust	Yes	No	No	No

As nouns, upper class and upper crust are synonyms defined as:

upper class and upper crust: the class occupying the highest position in the social hierarchy

upper class (noun)	upper crust (noun)
the class occupying the highest position in the social hierarchy	the class occupying the highest position in the social hierarchy