Fable is inappropriate for the vast majority of tasks (yes, including yours!), and it is quite obnoxious. Opus 5.5 writes concisely and almost sounds like a human.
Ok I bite, for what concrete tasks do you get better results with Opus than with Fable?
Because if true, I see no reason to spend that extra money, but for me Fable clearly outperformed Opus on complex tasks. (Including giving the same task and compare results)
IMO the default style of Opus 5.5 is still terrible but it is way better than 5.0. If you tell its styling is horrendous it usually rewrites in an actually quite good form.