Real benefits of RHEL over Fedora?

[Alex]

Hi,

I'm in the process of building a mail server and considering CentOS,
fedora, or RHEL. If I chose the RHEL option, I would probably choose
the minimal support level.

This server will be performing basic mail functions, including
postfix, dovecot, spamassassin, and webmail/squirrelmail. It will be a
moderately active server, delivering as many as 80k emails per day.

For something like this, will there be a great performance benefit to
using RHEL over fedora? Is the kernel that much different that it
would make a significant difference? Both would use ext4 for the
filesystem. Both would use the same spamassassin and postfix
versions...

Are the benefits to using the KVM/qemu virtual machine features on
RHEL 6.x that much better than what's available in fedora15?

I recall reading that CentOS is having trouble keeping up with the
latest RHEL. Is this currently a problem? Any input on whether future
updates will be delayed as well?

Thanks for any ideas.

Alex