Mystel is more populated and most of the new players start on that server, as far as i know. Killian daily population is mostly people who's been there for years and their parties are closed to new players. This, unfortunenately, includes myself most of the time. I used to help out new players to get them into endgame in the past but i have no time now. I just go with people i know, when i do stuff to rush down my daily farm.
If i didn't already have so many friends in Killian, i would switch to mystel right away. It takes forever to find a party, or even a last guy to fill your 4/5 party in this server even in peak times. And now it's just worse because half of the level 65 players will be fine with hitting a useless BAM in open world for 15 mins in 3-20 fps.
Other half is probably doing LKNM x4 with 10 characters in whole evening.