The general idea is: Use a package if you plan to exchange a lot of projects with a collaborator; use a folder if you mainly keep things on your own computer.
I do collaborate sometimes, but I don't use packages, because it holds no advantages for me. I just ZIP the folder and send it. No difference with ZIPping a package and sending it. I prefer folders because I prefer to have Finder access to all files.
A package is a folder, with the only difference that the Finder "sees" a package as one file; you can access the files within a package only via a contextual menu (control- or rightclick on it). But were you to save a project both as package and as a folder, they would contain the same files and be of the exact same size.