The Linux kernel has routing, assorted network capabilities, and firewall capabilities, and KVM. You can also harden it easily under CIS Security. If ClearOS is marketed so much with HP, what kind of hardware do you think it will work best for ?

Sounds like another Oracle UEK alike, and unless you want to pay for the extra service it offers to HP, I don't think it will be better than Linux. Perhaps the web UI is what entice you ?
Besides what makes the server really useful are the tools, the kernel is often not the most important thing despite it is the brain of everything. But a brain without interface is rather useless too
Just my take though.