Not sure, what you are saying but:
Redhat
only has trial versions available for
free. These
expire, if you need a redhat version (no trial), you have to
pay.
Fortunately, there is a
community driven free release of the redhat (based on redhat's sourcecode packages), which is practicly a clone, called
CentOS.
The same packages, same updates, same stability etc etc.. only called differently, and no support by mail or phone, just forum.
www.centos.org
But ok, you ask what to use RHEL5, Fedora 15 or RHEL6 or RH9?
Apple vs oranges situation here.
Fedora 15 is a testbed for Redhat releases, thus not stable by design compared to EL.
EL5 vs EL6, use the latest ofcourse (longer support in updates, newer stable software, improvements in redhat tools, etcetc)
But you cannot compare Fedora with EL releases, since their usage is different (also development method), and targeted audience.
EL for top stability, Fedora for expermenting new techniques.
Fedora release cycle is compared to EL, very fast. Suppport for updates etc, are compared to EL, short.
Oh, and DO NOT USE RH9, i repeat DO NOT.... unless you want to be very insecure, use very old programs etc etc..