I wouldn't say one is better than the other.
They both have high levels.
I would choose the nation your friends/guild are in.
Procyon has allot more player at the minute, so picking this will mean it will be easier to find partys for high level maps and boss hunting, ect. It also means you have the advantage of numbers in wars.
Capella has less players at the minute, but it still isn't very hard to find people to party with tbh. Especially if you join a capella guild.
Personally I would choose capella cause it is a better challenge and you get allot more honour per kill
.