IBM Case Manager is actually a product that sits on top of the base FileNet P8 components and ILOG JRules. It uses the underlying P8 Content Engine and Process Engine to manage the content and workflows respectively. So even with IBM Case Manager, you would still have FileNet P8 and ILOG JRules in your solution, but they would be hidden behind IBM Case Manager to some extent.
Apart from some improvements in the installation and deployment process over basic P8, the biggest thing that the latest versions of IBM Case Manager bring to the table is the ability for a knowledge worker to determine what parts of the workflow need to be completed on a case-by-case basis. Traditionally, workflow designs in the P8 Process Engine have been static and could not be changed at runtime. They still cannot, but Case Manager puts a layer of abstraction over the Process Engine to let distinct process fragments still be statically defined in the Process Engine, but have the overall process for a case be dynamic and determined at runtime by a knowledge worker.
That being said, if the project is for a process that does not involve knowledge workers, but only the integration of systems (eg. Credit Card Approval) then base P8 and static workflows with ILOG JRules could a better fit.