Claude, ChatGPT or Gemini for long-form editing: what are people using?

Some background. I lead SEO content at an agency and I try every writing tool that ships. The long-form jobs I care about are pillar articles and white papers, roughly four to twelve thousand words, where the edit is tightening, fixing structure and enforcing a house style, not writing from scratch.

Things I have noticed, none of them numbers:

• On a whole-document pass, the later sections get less attention than the early ones. The first third comes back cleaner than the last third.
• Asking for "improve this" gets rewriting. Asking for a list of suggested changes with the original sentence quoted gets editing, and it is much easier to review.
• Tone drifts toward a generic voice unless I paste two paragraphs of the client's real writing as a reference.
• The consumer apps and the API behave differently enough that I stopped comparing across them.

What I would like to know from people who edit long pieces: which one do you reach for and why, how do you split the document, and do you have a repeatable test? I do not trust my own impressions after three drafts.