My textbook defines assez as "fairly, rather" and plutôt as "rather". In the sense of meaning "rather", is there any difference between these two adverbs? Are there times when you would use one but not the other? Do they belong to a different register?
"Assez" conveys the idea of 'enough' but it can also be used interchangeably with "plutôt" when "plutôt" is used in the sense of 'rather'.
"Plutôt" conveys the idea of 'rather' as well as 'instead' ..
Hint: Look up "plutôt que" ... plutôt que de faire quelque chose --> instead of doing sthg
Check out the Larousse Chambers dictionary entry, that should clarify it. Larousse Dictionaire