It is good learn / study multiple areas in IT field.
Core DBA is a not subset of APPS DBA.
In APPS DBA works, You cannot explore the full potential of Oracle RDBMS. The APPS is fully developed, configured by Oracle. But with APPS, you will learn other areas such load balancing, Oracle forms, reports, WEB tier configuration, apps configuration etc.
Saary and perks depend upon the skill you show up in the job.