Choosing what to Write Next

Starting off with a question again this time, and it's a pretty simple one: how do you choose what to write next?

I think most writers don't have just one idea at a time.  There are some who write only one book, or one series, or what-have-you, but it seems like most of us have a bunch of different stories bouncing around in our heads.  I know I'm no exception; I've used the phrase "I keep universes in my head" more than once.  (Though I usually preface it with "I'm a writer" so I don't sound completely insane.)  But when I see writers talk about their processes, most only write one thing at a time.

For some people, what they write is partly determined by a contract.  They need to produce some number of books for some series by a certain time, so they do.  Or they miss their deadlines, but that's another entry, one I'll write if I ever have a deadline to miss.  I'd call these people fortunate, both because their books are selling and their publishers want more, though I don't know if they'd say the same.  It seems like it would be nice to know what you're writing next, but what happens when you get struck by that thing you just have to write?

Which brings me to the next possibility - writing something you just plain have to.  I've read about this one a lot.  There is something special about the writing compulsion being too strong to ignore or delay.  I think the closest I've hit for that was book eight on this list, which I plotted in about two weeks and started writing right away.  I thought I'd finally figured out how to write something I'd been wanting to write for years, a college story, but I crammed in everything else I wanted to write and it came out kind of a clusterfuck.  I haven't looked at that one since I finished it and I haven't felt like I absolutely had to write something since then.

(I'm not trying to be down on myself in every paragraph, I swear; it just comes naturally.)

Part of why I'm contemplating this is that my plotting is actually going well, and I have two stories I'd like to have ready to write by the year's end.  However, I'm not sure how things are going to go between now and then, so I don't know which one I'll want to write next.  Both have a lot of things in them I like, and I still want to write both of them, but I don't feel any great draw or need for either of them.  I think one has a much higher chance of selling based on the premise alone, but I try not to make decisions based on that.

To be fair, this has been a very difficult year for me on many levels, so I guess I can understand it being hard to drum up enthusiasm for anything.  I'm just hoping neither plot crashes and burns before I can get them into workable shape.

So.  What about the rest of you?  How do you choose what to write next?  Have you had the idea that drove you mad until you got it onto the page?  Those of you who've had contracts, how did that affect your desire to write the next book?  And is anyone putting odds on whether either of my plots will be ready by the year's end?

(Yes, I'm joking, because really, who'd bet on that?)