If you feel more motivated about your new story at the moment, you can absolutely start writing that one. If you manage to plan out the other story in the meantime, you can always go back to it, or work on it as a side project.
I’ve been there, I still am, because I have a story that I want to revamp since last year, but I just can’t decide how to do it. However, I wrote a new story instead (almost completed) and I really enjoyed it, so I’m glad that at least I did something 