Whats the difference between Site Admin and User Admin?

The Site Administrator is the main account holder for your hosting account -- you login here to manage your files, email, database, and so on. The Site Admin can create user mail box accounts. Then, these mail box users can login as "User Administrator" and set up mail forwarding, autoresponders, and other email related tasks.
To summarize, you get both, a site admin account to manage your and the ability to create user admin mailbox accounts.