Generally EU servers have a higher population, certainly the German ones.
But in this regard, don't look at the population but in first place to were you live. Should you go to EU servers, you will have a higher ping value (lag and you will see things a bit later, thus meaning you will get frustrated) and when you will play, many of the EU players will be sleeping or just waking up or logging out, due time differences. This means that you won't encounter that many players eitherwise. (If you're from America ofcourse).
So what's better? If you're from the America as a whole, go play on the US servers. If you're from EU go play on the EU servers.
Recently GF is merging servers and you never know, a cross US-EU server might be coming, meaning you will encounter many players.