Which of these should I write first?

I'm debating between two stories. The first is a murder mystery known as "Crystal Cove" which the prologue for is already complete (Under the name "Twilight Cove" in Answers), hoặc another story I haven't titled yet about a young man who travels through the US in the 1800's and has adventures. While I will write both, I just want to know what to write first. The một giây idea is still in the brainstorming stage, but I have a good idea where to go with it. So what do bạn guys think?