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As a Red Hat Certified System Adminitistrator, I would recommend CentOS as the best version for preparing for the RHCSA exam. CentOS tracks the RHEL releases, without the proprietary software. Fedora is another alternate to exam preparation. If you are considering SLED certification, I would go with openSUSE. I wouldn't go with Ubuntu, as the desktop edition is too far removed from the system administration world. Unless you are going to manage an Ubuntu server, then Upstart adds confusion to daemon management. It is nice to learn, but not part of RHEL, or SLED.
Ubuntu is good, but Red Hat has a certification program. If you want to save some money, download Fedora and then use it to take the Red Hat certification exams. Even if you don't have that much experience, it will help you to understand how Windows fits into a Linux world :P