Hello to you!!
I'm quite new myself, I've been around for 2 months now and I've figured only a very small number of things.
1. Recursion rules!!!
lol I mean this in all seriousness, not to start a server war, but just from what I know. In terms of population, I think that Recursion and Syntax have a good mix of players, vector is designed for the hard-core pvp'er in mind.
Recursion never has pvp all the time, yet it's always good to be witness to it when it does really kick off (like I've been finding out the last couple of days by acting as a rezzer watching the fights)
I guess it really depends what you want to do though. Like RemagDiv said, if you really want to go in just for pvp then go on vector but be aware that people can (and will) attack you once you're lvl 16. If you want to do on a more casual basis and when you want to, then go for either Recursion or Syntax. People do pvp on them all the time, yet the nice difference is that you don't have to if you don't want to.
Hope that helps a little bit,
Shadow x