Having been on several servers I can say they're mostly the same with a few minor exceptions to population.
Honestly I think the fewer amount of people are on a server the better. You only need 18 people for an alliance and 6 people for a party. Most events you don't need more than 18 outside of a few things where you have a seperate group kill adds/spawns etc.
tl;dr: *** waiting for MMM/having every camp taken/etc. Find friends on the server you're on and play the game. If you have to many people on your server, move to another one. If for some reason you like 3k+ players even though you'll never have the ability to know and talk to all of them and the only benefit I can think of is a cheaper economy, move to a crowded server.