Tender Is the Night is uncomfortably autobiographical, written after Fitzgerald's wife, Zelda, was institutionalized. Though it begins with the story of Rosemary, an actress on vacation, hopelessly attracted to Dick Diver, a married man and successful psychiatrist, the story changes, without transtition, to focus on Dick and his wife's unsettling past. Rosemary fades almost completely out of the story while Fitzgerald, vicariously through Dick Diver, explores his coming to (or failure to come to) grips with ageing, his marriage, postwar stress, and the fear that ultimately his promising career would fail. Fitzgerald literally fulfilled his prophecy and never published another novel.
As with most Modern American literature, Tender Is the Night is a depressing story. We witness the dissolution of marriage, man, and find the Lost Generation ultimately just that--lost.
It's been several years since I read The Great Gatsby, but if memory still serves, Tender Is the Night is more captivating and, in my opinion, the better of the two.