best practice either create multiple databases or multiple users
Databases use server resources: CPU, RAM, disk I/O.  Unless your server is physically partitioned (e.g. Solaris Zones) or you are using VMs it is a really bad idea to host multiple databases on the same server.  You are just creating an environment where contention will be rife and tuning will be even trickier than normal. 

Oracle RDBMS has always been designed to host multiple schemas in the one database, and for different applications that is the way to go.  This is one of the main differences between Oracle and (say) MS SQL Server or MySQL.