Even in PDF form I might prefer smaller volumes divided by subject, as particularly large, dense documents can start taking frustratingly long to page or search through - but PDF splitting software can solve that.Depends what format I'm buying it in.
In PDF, the single volume for ease of searching. I do essentially all my game preparation, as player or GM, on the laptop, so PDF is my primary purchase these days.
If hardcopy, two volumes, because binding an 800-page book so that it lasts is expensive. The pricing you list in the question suggests that the binding for the single volume won't be sufficiently robust.