From what I heard, the XMS2 CAS4 doesn't guarantee Micron D9 chips. Which are chips, that are of high quality, and great for overclocking. Nagasama got those. But now, it's a hit or miss. Where as the HZs are guaranteed, and they are excellent for overclocking. It's only that Nagasama got a bad set. But normally, they're not like that. So I would go for the HZs, if I were you. Although the XMS2 CAS4 are not that bad.