Deciding What to Work on Next

I'm on the second round of edits for SELLING HOPE, and *should* have them back to my editor this week.  But then, what next?  I've been researching two stories simultaneously, one a middle-grade road trip tale, the other a nonfiction picture book.  I've concentrated more on the mg story to this point, because, well, it's what I know - I lurve middle grade historical fiction.  (This one is half contemporary/half historical.)  My heart is in this project, but I'm currently struggling with the plot. Sometimes that can be good - some of my best solutions have come only after wrestling with the plot.  And authors are constantly told that writing 3 or 4 books of a similar type (ie, middle grade historical) is smart in order brand oneself.    

But my head says that the picture book can be researched faster (yes, even though it's nonfiction) and I have a very good feeling about its marketability (in other words, I think publishers will really like the idea.)  And it's not that I'm totally emotionless about this book - far from it.  The idea of it, and how I picture it being executed, gives me that itchy writer's feeling of "Yes! That must be written!"  And, too, maybe it would be nice to change gears for awhile - you know, get the sparks flying from a different piece of flint.     

So - which one to work on next - the heart or the head? The sloppy, messy, heart-driven book, or the practical, marketable change-of-pace?  How do *you* decide what to work on next?  And by all means, voice your opinion on my next project in the comments section!  Please?  ;-)