CentOS 5 is built from RHEL Source. Other than the logo and the different name, CentOS 5 is built from RHEL’s SRPMs (source RPMs).
CentOS will always be a little behind because of the current nature of the releases. But regarding security releases, CentOS is right on top of things, no lag there.
So, is there a difference? Not really, CentOS is a stable enterprices level clone of RHEL that just works.
So why would anyone buy RedHat Enterprise Linux? If you purchase RHEL, you are purchasing the support for RHEL which means that you get enterprise level support for it.
What about CentOS, is there support for it? CentOS has excellent documentation which anyone can use. Also any documentation for RHEL 5 is also applicable to CentOS 5. In addition there are mailing lists and newsgroups which you can go to for support.
All in all, they are both very stable, and well liked linux operating systems. If you need the support, gte the RHEL. If you are a comfortable with administrating a server yourself, then grab a copy of CentOS and save a little dinero.