Actually, I am looking in terms of usability and business purpose. so we can use either of them.
site publisher is used for publishing websites and e studio means to enroll in some course or program which is distance based.
I believe Sitepublisher is also specific to Documentum. I'm not familiar with eStudio, but I know its not Documentum product. So if you have existing Documentum Content Server, you are limited to Sitepublisher.