From the release notes VMware vCloud Director 5.1.2 Release Notes: "vCloud Director is now supported on Red Hat Enterprise Linux 6.3"
So, I guess I should go with that version to follow what VMware says. But as JayhawkEric said, there's probably nothing too major from the 6.X release that would cause an issue that I've heard of. But I think I'd probably follow the 6.3 for production, 6.4 for lab setup. Would be great if anyone has anything more about if 6.4 has got the VMware green light for production use.