I am not an expert in Xamarin. I only have spent 6 months on this programming platform. Thus, please feel free to response this post.
My frist preference is to Shared Projects. That is easier to manage. I can share xmal files across each project. That is simple, I think that is good to use in a small and one-off project.
However, if you have more focus on reuse, I think you choose PCL. That is more like a standard control library. You can use the controls in the library easily! Also, you are building a larger scale project, I think that is safer to use PCL, because PCL is more self-contained.
I think using PCL or Shared Projects is depending your project scale and goals.