The main difference between EDR and EDR-MDS is the master data aspect. EDR is the obvious choice for data-sources which you have no way of mastering, i.e. 3rd party Business Object sources.
EDR-MDS add the following
- Field and value based mastering (with write-back to all sources)
- Internal persistance (via the EDR-MDS backbone)
- Bulk-data dump
- delayed writeback-queues for handling off-line sources
- full resynk of sources
- Object history change and audit log