All Linux distros have some differences... But essentially, they all perform the same operations, albeit with slight desktop and/or command-line variations.
CentOS is a good, (free) RedHat-based distro, whereas Fedora is based on a debian system. When I first began using Linux I used Ubuntu (another debian based system, similar to fedora). The reason for this was for its ease of use - particularly in terms of software management and graphical interface.
Try the free clone of red hat first --> CentOs v5 --> http://mirror.centos.org/centos/5/isos/ If you don't like it, then go for fedora. If you're committed on learning the command-line stuff, rather than tinkering around appearance and software etc, then it really doesn't matter in my opinion.
Otherwise, you may need to buy a proper Redhat OS and/or pay for its support. In a big corporate, where uptime and stability is key - this is where the commercial grade of RHEL comes to the fore.
Even with CentOS which is basically RHEL, the support is not as good Red Hat themselves.
Edit:- Excuse my last post, Fedora is actually sponsored by RH, (and is not debian-based)!