oooh the religious war that is linux distributions

Picking a linux distro for your computer is a personal choice, use whatever works for you.
However, picking a linux distro for a server that needs to "just work" and run only one task, and sit largely ignored while it's doing it..... that's not a personal choice, it's a business one.
If you want a free distro, pick Ubuntu, if you want to pay, pick RHEL.
It's kind of that simple really. You may
want to run it on centos, but it it falls over due to a yum update, do you really want to spend time fixing it becuase zimbra's support team dont know the problem?