The only difference is the URL displayed.
Let’s say your primary website is yourdomainname.com:
- If you add an addon domain, yourdomainname.net and create a redirect to yourdomainname.com, the visitor, when looking for “yourdomainname.com”, will be entirely redirected to a different address (and see this address in the browser).
- If you park the domain yourdomainname.net to yourdomainname.com, visitors of yourdomainname.net will see the same website as yourdomainname.com (with all the latest changes), but in the address bar, they will see yourdomainname.com.