Well, first off, since NewEgg doesn't make RAM, you should take the manuf. number and go hunt down the RAM makers on the web and read their white-papers for all the technical nitty-gritty...
Second, the difference is that one's more dense than the other.
Sorry, couldn't resist.
Anyway, low-density chips usually mean that the chance for a "missed charge" is lower than with a higher-density chip, because the cells are further apart. However, higher-density chips run faster and are more efficient.
That's a REALLY REALLY REALLY thick layman's explanation...but you get the picture.