These adjectives, derived alike from a Latin word meaning "full," are interchangeable. Each refers to an overadequate quantity, an abundant amount: "The supply of food was plentiful (or plenteous)" Plentiful is usually preferable to plenteous because it is less "literary" and therefore more easily understood. Other words that could be used in this context include copious, ample, and profuse.
Dictionary of problem words and expressions. Harry Shaw. 1975.