Well I wouldn't spend all that money just on my say so. The Windows beta is not as advanced as Scrivener 2 but it still looks very good. Whenever I speak at writing schools I find most would-be writers use Windows which is what you'd expect - it's far and away the most common OS. Among professional writers I suspect the Mac is more common but still a minority. But I don'y know for sure.

As I say elsewhere here on Windows I'd use OneNote and on the Mac Evernote. But Evernote works on Mac and Windows and phones and tablets too. So if you want to chop and change it would be ideal. If the current project continues on Scrivener 2 on the Mac that side of things will move to Evernote (which isn't as good as OneNote but still pretty useful).