How much utility would you get out of an unfinished book? Have you factored that into your expected utility for the project?

I think you should go for an episodic format, with pieces that could be publishable by themselves even if you don't achieve your primary goal. Like Freakonomics. Big monolithic narratives like The Better Angels of our Nature are great if you're an experienced and (more importantly) well-known author who can easily recruits lots of assistance.