It's one game server, just two different patch servers. The main server is US-based. If interested in joining a tribe, please click the link in my signature. Welcome to Xsyon.
I dont think there is a US and EU server, only a US server as far as I know. But its been over a year since I created a character, I could be wrong, I have been before.
But the population on the server is pretty small, its not WoW by any stretch but its decent, probably about 80-150 during peak hours down to 10-50 during off hours. Weekends it picks up a lot, maybe 2-3 times those numbers.