Well... thats a hard question. If you can get both, do it .But if you can only afford one, then go for blue moon because is better for dungeon. The problem with magnum/arcana is that more gear you have, worse is the effect, not because the badge itself but the skill. Triple shot doesnt get any buff from the gear besides the general damage buff and at high lvls of gear, the damage difference between spending time lauching true shot instead of dark shots is huge and can be a dps loss, especially if in short terms fights (thats why blue moon/colossus is better for dungeons). Btw, i use my arcana