sneak peek vs preview

sneak peek and preview both are nouns.

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

As nouns, preview is a hypernym of sneak peek; that is, preview is a word with a broader meaning than sneak peek:

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

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

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