High pop servers are like big cities: there's a ton of people of all types to socialize and do stuff with, but you have to deal with crowding (hunts, etc) and it is harder to get noticed. Oh, and real estate is at a premium, so good luck buying a nice house.
Low pop servers are like small towns: there's not a lot of people, which can make for a close community but also alienate some folks. It can be harder to find people to do stuff with. But there's little worry of having to deal with crowds, and the real estate is affordable and available.
I'd personally pick a higher population server; it is like having the perks of living in a big city without having to deal with obnoxious neighbors and traffic.

But if you may ever want a house, I'd stay away from the
highest pop servers (Balmung, Excalibur, Gilgamesh, etc).