Consider all the other books that will be displayed alongside yours. To stand out among them, your book’s cover should look different. This will be more important in a bricks-and-mortar store than online.

Will the book be placed among business books? Perhaps use the turquoise cover. Will humorous books be its neighbours? Use the darker cover. The aim is to stand out in competition.

Personally, I prefer the darker cover for the contrast between text and background.