finishing vs completion

finishing and completion both are nouns.

Word	Noun	Adjective	Verb	Adverb
finishing	Yes	No	No	No
completion	Yes	No	No	No

As nouns, completion is a hypernym of finishing; that is, completion is a word with a broader meaning than finishing:

Other hypernyms of finishing include closing, culmination, mop up, windup.

finishing (noun)	completion (noun)
the act of finishing	a concluding action
a decorative texture or appearance of a surface (or the substance that gives it that appearance)	(American football) a successful forward pass in football