deterioration vs decline in quality

deterioration and decline in quality both are nouns.

Word	Noun	Adjective	Verb	Adverb
deterioration	Yes	No	No	No
decline in quality	Yes	No	No	No

As nouns, deterioration and decline in quality are synonyms defined as:

deterioration and decline in quality: process of changing to an inferior state

Other synonyms of deterioration include declension, worsening.

deterioration (noun)	decline in quality (noun)
process of changing to an inferior state	process of changing to an inferior state
a symptom of reduced quality or strength