I tend to disagree with both points.
> The W3C is for web authors. It presents a more stable recommendation and provides advice (based on research) for authors.
Web authors usually use MDN instead, because it serves that purpose in a much better way. (Note that despite the name MDN is not Mozilla specific but a cross-browser resource, and that Microsoft and Google recently joined MDN.)
> The WHATWG is for browser vendors. It's in a constant state of flux as new changes get proposed and those proposals get changed through implementation.
The W3C recommendation is no different in that regard, they also describe stuff that's not fully implemented in all browsers yet. But the WHATWG version is more up to date, so you'll notice much earlier that the new feature you want to depend on will be abandoned or changed.