Ussually the first few servers have the most people coming from beta etc and they tend to be quite serious and stay for a long time in the game. With the pace the publisher has been releasing servers the first few servers is your safest bet. It just means you will not be able to start at the same point as the rest as they are already lvl 70. But you can still catch up in time ofcourse.

Later servers will probably be really busy at the start but they are more likely to lack population after a while. But if I saw correctly, the chinese server has some cross server rankings etc. So as long as you find 1 active group you will probably have a good time untill it possibly gets released in the west.