I think the thing about PlanetSide's factions is they're so perfectly designed and equally balanced in terms of appeal that the only real choice that a person makes when choosing one is their taste. NC typifies the whole "good guy, rebellion" thing like the Rebels in Star Wars, which is popular for those who like to think they're the good guys. TR typifies the whole "galactic empire, evil but-good-depending-on-your-perspective" thing that has the following of people who look at it as badass, while the VS is appealing due to all its funky technology and alien art style, which is also very popular - the whole alienware computer designs and the covenant.
I hope the TR have a zerg in PS2. Fantastic source of kills for the hardcore NC