Now that you mention it what is the whoe difference between Aether, primal and crystal player base or server wise.
Based on current Party Finder, very little.

Crystal is more active than Primal, Aether is slightly more active than Crystal.

We expect Aether to always be a little more popular, but Crystal has been picking up people who moved to Aether, said "this isnt much better and I'll never have a house", and bolted for a smaller server.

Crystal also has very strong cross-server activity. It wont have as much of the top-end raid scene, but itll have a raid scene.