I own both. They are very different. The European rail map is very useful for long distance travel, route planning etc, whereas the Eisenbahnatlas is very detailed and for Germany it is a very large book - not something I would ever take on a journey.
The European rail map is available on CD as a PDF so I carry this on my phone.
The latest editions of the Eisenbahnatlas are only available as books. There is an earlier version available on CD which I own. I can then select which 'bits' of Germany I need.
You need to define what your real requirements are.