Fedora 13, which hasn't been released yet will be the basis for Red Hat 6, but, if your using the OS to study for a certification exam, I would use Centos, since I'm sure the exam would be based on the current version of Red Hat. I've never taken one of these exams, so I can't tell you exactly what the advantages are for using one over the other.

There will always be some differences between what is free and what they sell to enterprise customers, if you want to guarantee that you are interacting with RedHat's enterprise distributions and their complete suite of tools then you should try their 30-day trials of both server and desk editions (http://www.redhat.com/rhel/details/eval/).