Hey Ingrid,
Thanks for reaching out!
There is NO difference, and in fact, one thing I would like to clarify are that Events Calendar Pro is not a “pro version of” The Events Calendar.
The Events Calendar is not just free, it’s the “Core” plugin. So it will need to remain active and installed even if you add Events Calendar Pro or any other plugin here on http://theeventscalendar.com.
Events Calendar Pro and our other premium plugins are “add-ons” to the core plugin The Events Calendar.
So if you buy Events Calendar Pro, then on your site you will have The Events Calendar AND Events Calendar Pro both activated and installed on your site.
Does this make sense, and hopefully help clarify things a bit?
If so, then when it comes to organizers, those are a “core” feature; The Events Calendar handles Organizers. All of the other plugins just “use” that core organizer type.
Let me know if this helps!
— George