Honestly you should write whichever one calls to you the most/whichever one you are most passionate about. Writing a book is a LOT more work than you probably realize and you are much more likely to finish it if you are writing about something you LOVE than something you are MEH about but think will be interesting to other people.
I second this! As someone who has made her living at writing for almost 15 years, book-length work is incredibly difficult to do. There will be so many ups and downs in the process of writing, and it will be so tempting to quit at many different points in the process that it should be something that you are really, truly passionate about.
You know the title is supposed to be the last thing you write.
I call bull on this. I know many, many writers who have books and articles that have started with a title. Sometimes it's a character, a scenario, an image, and sometimes it's a title. Honestly, the majority of my books have come from a single, first sentence popping in my head and then I build everything on from there. I think I remember reading (maybe in the Paris Review?) that Faulkner started with the title for The Sound and the Fury (an allusion to Shakespeare that he thought would make a good title), though I might be off on that one. At any rate, there's no process that's wrong, and no process that's right. (Though there are wrong and right outcomes!)