What is the difference, if any, between Web Deployment Tool 2.1 and Web Deployment Tool 2.1 for Hosting Servers?

Microsoft's Web Platform Installer lists "Web Deployment Tool 2.1" with a release date of 4/11/2011, and "Web Deployment Tool 2.1 for Hosting Servers" with a release date of 4/8/2011. As far as I can see (by clicking Add, then Install), the contents are the same: the deployment tool itself, SQL Server 2008 R2 Management Objects (Dependency), SQL Server Native Client (Dependency), and SQL Server System CLR Types (Dependency).

Are these actually different, and if so, how? (And if not, why list them both?)

You're correct, the contents are the same. However the 'for Hosting Servers' package will install a number of other dependencies such as IIS, if not already present.

Also, I would add: " for Hosting Servers" option installs the "Web Deployment Agent Service" which is required to be the receiving server when doing an over-the-wire migration using msdeploy.exe.