I think the only difference is the paid support.
Well, it's a bit more than that.
[li]The binary RPMs from Red Hat are the ones that hundreds of hardware and software vendors certify against. That is an important difference when you are troubleshooting.[/li]
[li]The control of the RHEL roadmap is in Red Hat's hands. There are only a few ways to influence that roadmap. You can work in Fedora, as many CentOS people do, or in other upstream projects directly, and let that trickle down to the next RHEL release. Or you can be a customer and have your product managers work on it for you.[/li]
[li]When it comes to working with upstreams on the behalf of customers, Red Hat has the longest and greatest record in this industry. If you are prepared to work on your own behalf in any upstreams that matter to your business, then you have one way to keep control of your future. Another is to have Red Hat do it for you.[/li]
Interesting that the question of "RHEL or CentOS?" is often discussed on both sides as a matter of cost. When you look at it further, you find there are legitimate reasons to decide either way, depending on your needs and resources.