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 for raids because in long fights is better for me than bluemoon/colossus. So... thats all: the safe option is bluemoon, the risk option is magnum and de pref option is both.
Hopefully this will be helpful.
PD: Dont rush the fuse version for any of those badge (magnum->arcana ; bluemoon-> colossus) because the extra effect from eternity is almost useless.
PD2: Magnum/arcana will be buff with the awakening patch because the change to triple shot (1 shot with 3 hit instead of 3 shot with 9 hit for trigger true shot) so, even if you prefer blue moon now, i recommend you to get magnum in the future.