sooner vs rather

sooner and rather both are adverbs.

Word	Noun	Adjective	Verb	Adverb
sooner	No	No	No	Yes
rather	No	No	No	Yes

As adverbs, sooner and rather are synonyms defined as:

Other synonyms of sooner include preferably.

sooner (adverb)	rather (adverb)
comparatives of `soon' or `early'	to a degree (not used with a negative)
more readily or willingly	more readily or willingly
	on the contrary
	to some (great or small) extent