The Kingston 4GB DDR3 KVR1333D3S9/4 seems like it's better solely based on the reviews on newegg... The only difference is the Kingston 4GB DDR3 KVR1333D3S9/4 has a manufacturer's warranty. The Kingston 4GB DDR3 KVR13S9S8/4 doesn't. I think the Kingston 4GB DDR3 KVR13S9S8/4 is an older version which would explain why it doesn't have a manufacturer's warranty and it's model # is S8/4 while the supposedly newer one is S9/4, that's just a guess/observation though.