If you feel strongly motivated to finish the book, then I'd do that. I think you'll be in a much stronger position as regards the pitch - you'll know your subject better, you'll be pitching it from a position of accomplishment ("I have written about..." is much more authoritative than "I'm planning to write about..."), and for these reasons you'll put yourself higher up the queue. There is also less risk for agent/publisher, as they know what they'll be getting. If they like your idea but would like to suggest changes, then they will. The main downside with this approach is really buying the time to complete the book - but it sounds like that might not be an issue for you.