Most HPC cluster installations I know (small to medium edu sites in Germany) use CentOS / RedHat. Source: am part-time HPC sysadmin
Interesting, I thought SUSE was king in Germany.
At least one NASA supercomputer runs Suse. A review of the TOP500 list would probably reveal quite a few Suse HPC systems.
Don't SUSE tout that they can do in-place kernel upgrades? So if you want to keep your iron running all the time, no interrupted experiments, no reboots; SUSE is the distro to go for.