De acordo com o link do TechNet , a diferença está na intenção :
As caixas de correio compartilhadas são para comunicação externa (ou seja, [email protected]), as pastas públicas são para colaboração interna (ou seja, [email protected])
Caixa de correio compartilhada
A shared mailbox is a mailbox that multiple designated users can access to read and send email messages and to share a common calendar. Shared mailboxes can provide a generic email address (such as [email protected] or [email protected]) that customers can use to inquire about your company.
Pasta pública
Public folders are designed for shared access and provide an easy and effective way to collect, organize, and share information with other people in your workgroup or organization.