1) CentOS build from the Source RPM of RHEL (from redhat)
2) RHEL get faster update then CentOS because, CentOS need to wait to get the source rpm from the RHEL repository (sometime centOS community come with their own patches, but there is very small delay when the Redhat Update)
3) CentOS free version of RHEL ( which means there is no license fee)
If you ask me, if you can get RHEL license free ( most of the planet server comes with free license), use RHEL. If you are going to pay for RHEL license fee more then 50+USD. I would use the CentOS.
NB: CentOS is RedHat Enterprise without the trade mark / copyright. The same functionality, the same OS; though RedHat Enterprise typically has a faster update cycle especially RedHat Enterprise 3 or CentOS 3