Pittsburgh of the South vs urban center

Pittsburgh of the South and urban center both are nouns.

Word	Noun	Adjective	Verb	Adverb
Pittsburgh of the South	Yes	No	No	No
urban center	Yes	No	No	No

Pittsburgh of the South (noun)	urban center (noun)
the largest city in Alabama; located in northeastern Alabama	a large and densely populated urban area; may include several independent administrative districts