At a high-level, the main differences are as follows:
1) From a technology point of view, in the model layer, OEPE doesn't support development using the ADF Business Components. OEPE supports EJB/JPA and POJO based models.
2) At a feature level, the Web Page Editor in OEPE is a limited to a Source editor and doesn't have the graphical editing capability that is present in JDeveloper.
If you share the details of the nature of the application you plan to build using ADF, we will be able to provide better guidance. Hope this helps.
OEPE 126.96.36.199 is now available that has support for ADF 12.1.2 and JSF 2.x. In addition, it has support for ADF Essentials, the free distribution of ADF. See this blog for details.
the whole idea of bringing ADF into Eclipse is to avoid such a question. Right now the ADF integration in JDeveloper is more complete than in Eclipse as there are missing areas we did not yet work on in OEPE (e.g. customization, personalization, ADF security). In the long term ideally it all boils down on team organization and the IDE only becomes the tool to create the artifact to be deployed in ADF libraries for later application assembly.
One thing that will help bringing ADF to Eclipse is adoption.