Ok so I did a bit of research and I found that lower CAS Latency is better, as well as memory timings. I looked at the Corsair Vengeance 8GB, it has a CAS latency of 9 and 9-9-9-24 timings. The 16GB Vengeance RAM had a CAS latency of 10 and 10-10-10-27 timings. So which one should I get? It doesn't really matter what size RAM I get, which one would I be able to squeeze more performance out of? Since the 16 GB has higher CAS latency and timings, will it still be better or worse than the 8GB kit?
Since the likelihood of you using up the 16GB's is small I would go with the faster kit. The reason I really like the vegeance kit though the 16 gigabyte one which I think I know the one you are looking at it has 2x 8GB sticks low profile is because its low profile. If you do anything beyond gaming I would suggest the 16 kit otherwise stick to the faster CAS 8 kit.
The performance difference is going to be extremely minor, depending on what you do, you would likely never notice a difference. That being said, the 8gb had better timings, I am sure they also offer it in a 16gb set.