It all depends on the application, and whether or not I was ordering anything else at the same time, saves on shipping. In general, I go for the good stuff from a reputable vendor- I've used Azzo, Memman, Crucial and most recently Mushkin, I like Mushkin a lot. Killer service, and they're local, so I pay no shipping.
Most people don't need the hottest top-of-the-line stuff, but quality shows, and is worth the extra $.