Dream Traveller sounds more catchy to me. It features two elements that tell us something about the novel, which is good. It makes me wonder who exactly would travel in a dream and how exactly that would work! Which is I good thing for a title, I think.
In My Dreams however, only really has one of those elements, in my opinion. "My" I never think really fits well in a title because we have no idea who the "My" is, so it isn't really something that grabs our attention! The other thing is, everyone has dreams, so that to me makes it even less of a hook.
