Since the two books are from two different genres, it is no surprise that the styles in which they are written differ - Susanna Kaysen's "Girl, Interrupted" is a collection of memoirs whereas Sylvia Plath's "The Bell Jar" is a novel, though autobiographical to a degree. From a literary point of view, "The Bell Jar" is the better written of the two in that its narrative has a smoother flow and it is rich with the same kind of literary techniques and symbolism which any reader of Plath's poetry would be accustomed to; "Girl, Interrupted" on the other hand is slightly more amatuerishly written. It is worth bearing in mind though that Plath's novel is a literary piece while Kaysen's is an autobiographical piece.
Plath's book is essentially a fictional novel however when one draws comparisons between Plath and her heroine Esther Greenwood, it is obvious that "The Bell Jar" is, to a large extent, autobiographical.
Nonetheless, Plath was an established writer - almost wholly of poetry - and this shows through in the book. Kaysen, on the other hand, was not a professional writer and, similarly, this shows through in her book. Although "Girl, Interrupted" is autobiographical, Kaysen's style deviates from this at times. She doesn't necessarily explore events in chronological order, sometimes jumping back and forward between situations.
The tones in which Plath and Kaysen wrote their books differ quite considerably. Kaysen has successfully injected some humour into a book on a fairyl serious subject; granted this humour is usually quite dark and almost sarcastic. "The Bell Jar" displays little - if any - humour and is instead very serious, a tone in-keeping with Plath's poetry.
Both have achieved great praise and acclaim but the truth of the matter is that if Kaysen's "Girl, Interrupted" were a work of fiction as opposed to a memoir it seems unlikely that it would have been nearly as popular due to Kaysen's erratic and often inconsistent style of writing.