I say you write what you can't get out of your head. Because if you don't, what you do write will be compromised by your inability to focus on it and your current lack of passion for the story.

My take is that the Prep helps you get to the place where you can be successful for NaNo. I think it's doing it's job if it's refining your ideas and helping you get focused on a story (even if it's not the one you thought it would be).

Just my two cents.

Best wishes,

Amy