selection vs mixed bag

selection and mixed bag both are nouns.

Word	Noun	Adjective	Verb	Adverb
selection	Yes	No	No	No
mixed bag	Yes	No	No	No

As nouns, mixed bag is a hypernym of selection; that is, mixed bag is a word with a broader meaning than selection:

Other hypernyms of selection include assortment, miscellanea, miscellany, mixture, motley, potpourri, salmagundi, smorgasbord, variety.

selection (noun)	mixed bag (noun)
the act of choosing or selecting	a collection containing a variety of sorts of things
the person or thing chosen or selected
a passage selected from a larger work
an assortment of things from which a choice can be made
a natural process resulting in the evolution of organisms best adapted to the environment