There's a whole lot of mis-information going on here. Primal is a EU based datacenter, and Aether is an NA based one. That's the biggest difference. It's recommended to choose a datacenter you're closest too for ping response times, unless you have friends on a certain datacenter. Choosing a server based on population depends on your preferences for playing FFXIV. Are you a loner who doesn't really interact with many people? A low pop server would be right for you. Do you enjoy interacting with people but don't want to be crowded? A medium pop server might be right for you. Do you enjoy interacting and socializing with a big group of people, RPing and/or seeing a lot of people around cities and hubs? a high pop server might right for you. Now the problem with high pop servers at least on Aether is most of them are closed off to new character creation and transfers (Balmung and Gilgamesh). Also note that the duty finder pulls players from across the whole datacenter, so that shouldn't really matter in deciding on a server.