Difference

On here, can anybody tell me the difference between the top left and bottom left sticks of RAM. I *think* the PQI is the company who makes it, but the bit where it says 4-Layer completely threw me out. Annoylingly, this site uses java pop ups for each item, so i cant link my current ram, but the quickfind code is MEM-512-3200-E, and its the top one out of the two. Will either of the first 2 rams work with my current one?

This is just a guess, but i think 6 and 4 layers is how thick the circuit board is. It seems like higher could be better, seeing that the more layers there are, the more expensive it is.