At the end of last year, SAP Employee Andreas Kunz wrote a blog post
What is OpenUI5 / SAPUI5?
explaining what is OpenUI5/SAPUI5 and also telling about the history of UI5 and the differences between the two versions.
Currently the main differences are some missing widgets (in a dedicated namespace) that are (mabye currently) not available in the open source version of the library.
Also the Theme Designer is currently not yet released as Open Source, but you are able to theme your app manually using LESS.