Oracle Reports generates static reports. Parameters can be defined by the developer at the time of development and cannot be added at run time unlike the Discoverer Report. Discoverer generates a much more interactive report. In Discoverer, the end user can define parameters at run time and also add mathematical calculations to the report.
It is easier to develop reports with graphs in Discoverer than in Oracle Reports.
So if the requirement is to provide an interactive report, we would go for Discoverer Report and if the requirement is to provide a static report we would be going for Oracle Report.