NA server used to have quite the healthy population. I'm guessing you couldn't go wrong with either one. Why not give them both a try and make a decision from there?
It all depends on your timezone and when you're available to play.
For example:
If you're in the US and can only play between 10am-5pm then EU may be more populated because that will be the peak time. If you can only play your evenings 5pm-12pm then there will be less players on EU compared to NA.