I prefer CentOS (http://www.centos.org) as a server OS. It is well tested, secure, and reasonably up to date. CentOS is very nice for the sysadmin: configuration is well thought out, handy utilities like "service" and "chkconfig", automatic security updates, etc. Plus if you need support, upgrading to RHEL is trivial.

Unfortunately I don't think CentOS is built for PPC.