The DDR3 1600 would be better but CAS 7 is probably very expensive compared to the much more common CAS 9 and you won't see any considerable/noticeable performance increase. If this is for a gaming machine you should know that no game see's any performance increase from using faster RAM.
The pricing is not similar. The 1333MHz kit has double the capacity of the 1600MHz kit.
Capacity is much more important than frequency and latency. The 1333MHz kit has a decent price but the 1600MHz kit is very expensive for it's capacity and I recommend against buying the 1600MHz kit in this situation. Are these the only two kits you're willing to consider?
Right now I am looking for 6GB/8GB memory stick with 7CAS or less. It could be in the 1333 series or 1600 as the difference is very small for its price.
Also, my budget would be around 50$ for the memory sticks.
I'm not sure why you must have cas 7, cas 9 1600 would be faster than cas 7 1333 although it makes close to no difference. http://www.bit-tech.net/hardware/memory/2011/01/11/the-... You would also be better off with ram at 1.5v. So get 2x4gb, 1600, cas 9, 1.5v. Of course the lower the cas and faster the speed is better if you can find them at similar price.
You won't find 6gb kits as there are no 3gb sticks. The triple channel kits are not for SB.