Prep the one that's speaking to you more, but don't feel that just because you Prepped it you have to use it for NaNo! One year, I prepped for a story that fizzled out around 10,000. In week 2 I started from 0 with a story I hadn't prepped and actually won. Just keep the other idea in your back pocket just in case and prep on the side if time and/or inspiration allows.