Is there any problem restoring database from 9G to 9i

Everyone, I need to make a copy of a database in 9G, and put it on another server with 9i. Is there any problem with that ? Is 9G database different than 9i ? The platform is Solaris. And what is the best tool for this ? Will I use RMAN to take a backup, then restore it on new server using RMAN again?