I can't tell you what to do as we all have different ways of handling our writing, but I bet you are not alone in doubting your story.. That happens a lot.

Also, you might consider doing preps for both stories. It isn't too late.

As for me, I knew nothing about my current prep story either. I even debated whether to do NaNo this year or not. Then the fun I usually have with prep and NaNo took over. The night before Oct. 1 I thought of a journey taken by a group. Other than that, I had nothing, zilch! But I am excited about it now. There's a good chance I won't be able to make the 50K in November, but who knows?