upper crust vs upper class

upper crust and upper class both are nouns.

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

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

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

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