StartUP with Xamarin CrossPlatform.

I am working for a Crossplatform app using Xamarin Studio IDE for that. I am dividing the code in different projects

1. Core (Business Layer)
2. Android (Android Application and UI Layer)
3. iOS (iOS Application and UI Layer)
4. Mac (Mac Application and UI Layer)

I am ok for all the platform specific project but I am confused with the core, which templet I should start with for Core Project.

Should I use the Generic Project (A project that can contain any kind of file) or I should create Portable Library (A C# Library can be used in Windows, MonoTouch and Mono for Android ?