preview vs sneak peek

preview and sneak peek both are nouns.

Word	Noun	Adjective	Verb	Adverb
preview	Yes	No	Yes	No
sneak peek	Yes	No	No	No

As nouns, sneak peek is a hyponym of preview; that is, sneak peek is a word with a more specific, narrower meaning than preview:

preview: a screening for a select audience in advance of release for the general public
sneak peek: An early view of an unreleased product

Other hyponyms of preview include sneak preview.

preview (noun)	sneak peek (noun)
a screening for a select audience in advance of release for the general public	An early view of an unreleased product
an advertisement consisting of short scenes from a motion picture that will appear in the near future

preview (verb)	sneak peek (verb)
watch (a movie or play) before it is released to the general public