Have never noticed a difference between Fable and Opus on coding tasks but design/architecture is night and day different. I usually don't bother with designs with Opus since it tends to screw things up, hallucinate, make bad decisions.

With (obra) superpowers, Fable writes a plan and delegates to Opus which works well. Fable is much better at sorting architecture in a conversation rather than me creating a giant prompt for Opus.