Where Claude is the better prose stylist, ChatGPT is the better planner. Ask for twenty premises, five ways to restructure act two, or a list of what a reader will predict about your ending, and it produces volume quickly - which is exactly what the planning stage needs, because you are going to reject most of it.
Strongest at: outlining, breaking a stuck plot, and generating options you choose between. It is also the most useful of these for the work around the story - the query letter, the synopsis, the blurb - which is a real part of finishing a book.
Worth knowing: its default prose is the tidiest and most generic on this list. Every sentence balanced, every paragraph resolved, no rough edges anywhere. That is fine for an outline and wrong for a scene, so use it for the shape and write the words elsewhere.
Best for: planning, plot problems, brainstorming at speed, and everything you have to write that is not the story itself.