Would it be vague to say “listen to your gut”?
What I did was run through the story super fast in my head. Take a few minutes to think up the premise and then rush through the middle part, straight to the endgame. This made me realize that the story I thought I had entirely planned out had massive hickups that I still had to work out before I could confidently start writing the code.
Another fun way to see which script feels better is to grab a moment from the middle of your story and write a couple thousand words around that part. It gives you a feel of your story without you having to think about the beginnings or ends.
There are many ways to decide, but these two have helped me. There’s also this thread that helped me as well.