They'll both work.
They're both free as in beer, and as in speech.
(at least the CentOS and WBEL compiles of RHEL sourcecode are)
RHEL 3.x / CentOS 3.x is professional system that will be supported with updates for at least 3 more years. RHEL 4.x will be supported even longer.
Fedora Core 3 will be supported probably no more than a year before you'll have to update again.
Jeff