There is a small percentage performance increase with the CAS-2.5 over the CAS-3 in some applications, mainly games. One other thing to consider is that the CAS-2.5 will run faster than the CAS-3 making it your best bet for speed and stability when overclocking. So if you want to wring the last bit of performance out of your system, get the CAS-2.5. Otherwise, CAS-3 memory should meet your requirements.