In my presently-on-hold manuscript (Manuscript A), there is a Thing the main character wants.  And in the course of the story, she makes a choice to do what needs to be done so she can get it.

In my presently-being-first-drafted manuscript (Manuscript B), there NEEDS to be a Thing the main character wants so that she can make a choice to do what needs to be done so she can get it.  It's the whole point of a story, right?  The protagonist's quest?

I've spent many weeks trying to decide what The Thing should be in Manuscript B.  I even opened it up for suggestions at one point.  And while there were many good ideas here and from friends and family, none of them have felt quite right to me. 

So, I continued moving forward in the draft, writing all the scenes that don't have anything to do with The Thing.  But I'm getting to the point now where I need to make a choice and go with something--even if it is a temporary something.  I'm going to get stuck if I don't.

In the last few days, I've come to the conclusion that The Thing that might be best for Manuscript B is the EXACT Thing that I was intending to use in Manuscript A. 

And, well, that just won't work.  I can't have two stories with the same Thing in them!  Which means I need to either give up on Manuscript A entirely, or change the Thing in Manuscript A to something different in order to use it in Manuscript B.

I'm feeling very uncertain about what will be the right thing to do, particularly as I don't know which, if either, will be more likely to sell for me, you know?