Personally I prefer GPT 5.5 writing style over Opus 4.8. It’s much more no nonsense and information denser.
Personally, Claude Opus (and in the few interactions I had with it, Fable) has been the far the superior experience. GPT-5.5 seems dumber and more certain about presenting me bullshit. Opus has better humor, and is less pretentious in its presentation. But this may all boil down to how the models react to my prompting.
What is without a doubt is that I wish they both were more intelligent – or maybe it is their wisdom I find lacking!