Which is better for a fiction writer?

On Writing. King's advice on story, dialogue and revision comes from a novelist, and the memoir sections keep you motivated. Zinsser focuses on nonfiction.

Which is better for business or technical writing?

On Writing Well. Its chapters on simplicity, clutter and usage apply directly to reports, articles and documentation. King's book is more about storytelling.

They overlap less than you'd think. Many writers keep Zinsser as a reference manual and read King for inspiration and habit-building. Reading both is common and not redundant.

Which is easier to finish?

King's is shorter and reads like a story, so most people finish it quickly. Zinsser is structured as a course and rewards steady chapter-by-chapter reading.

Which has held up better over time?

Both are enduring classics with over a million copies in print. Zinsser's usage examples show their age in places; King's anniversary edition was refreshed with new material.

Is one worth stepping up to if I already own the other?

Yes, if your needs differ. Add Zinsser if you write nonfiction and want method; add King if you want motivation and fiction craft. They serve different jobs.

Stephen King On Writing vs On Writing Well

Buy On Writing if you want craft lessons wrapped in a memoir from a working novelist; buy On Writing Well if you want a structured, practical manual on nonfiction prose. King's book is more inspiring and personal, with strong word-of-mouth from writers. Zinsser's is tighter and more instructional, organized around principles, methods and forms. Both are short, readable and widely assigned in classes.

Choose the King book if you want motivation and a storyteller's view of the writing life alongside the advice.
Choose the Zinsser book if you want a systematic, no-nonsense guide to clear nonfiction writing you can apply chapter by chapter.

Prices

Stephen King On Writing: $9.23 at Amazon (checked today)
On Writing Well: $15.99 at HarperCollins Publishers (checked today)
Difference: the Stephen King On Writing is $6.76 cheaper than the On Writing Well today