Motor City vs urban center

Motor City and urban center both are nouns.

Word	Noun	Adjective	Verb	Adverb
Motor City	Yes	No	No	No
urban center	Yes	No	No	No

Motor City (noun)	urban center (noun)
the largest city in Michigan and a major Great Lakes port; center of the United States automobile industry; located in southeastern Michigan on the Detroit river across from Windsor	a large and densely populated urban area; may include several independent administrative districts