There are usually more people on the good side. Certain classes are good or evil only. (If you want to play an evil race as a good class or vice versa, you can do a betrayal quest to switch cities. You have to do the quest between levels 13 and 17, and it's pretty long and involved.) At the early levels, it's hard to reach the opposing city and its adjacent zones. However, by the mid teens/early twenties, it's not difficult to go anywhere you want, and you can group with people from the opposing side easily. (It will always be hard to actually enter the opposing city, but you won't have that many reasons to.)