Arcana Robe Set is very nice for mass pvp but only if it has 120/120 fire/water resistance. Otherwise it is pretty useless, unless you want it for pve purposses. For small scale pvp, or for the times people gank you, if you don't have some support to back you up you won't like Arcana Robe due to less casting speed. The difference between the sets is about 100 casting speed.
But of course, s80 robe is the dream of all nukers, try to focus on that in the future.
Arcana Robe Set is very nice for mass pvp but only if it has 120/120 fire/water resistance. Otherwise it is pretty useless, unless you want it for pve purposses. For small scale pvp, or for the times people gank you, if you don't have some support to back you up you won't like Arcana Robe due to less casting speed. The difference between the sets is about 100 casting speed.
But of course, s80 robe is the dream of all nukers, try to focus on that in the future.
Good Luck in Olympiad in DC or Arcana Robes. Most of the top Nukers have S80 which is like an attributed DC Set (they stack all pieces with Attribute). They will do about twice your damage i.e. you nuke them for 900 they nuke you for 1800 in DC. You can fully attribute an Arcana Robe Set, but they will have a factorably higher Casting Speed. In other words, you will get plowed hardcore.
I agree with everyone else here. I have a few other comments too.
Because I'm not 80 yet I use my Major Arcana Robes A LOT for PvE! They work really well in castle instance where you may not have a recharger and they work well for raids (in these situations casting speed doesn't matter, only M.Atk. and MP, and Major Arcana lets you hit harder for the same amount of MP). Also I use my Major Arcana set A LOT for general exping as I can carry more stuff, I have slightly more MP, and it takes less hits to kill mobs (Major Arcana has the weight gauge increase). IMO if you have the money Major Arcana is a really good PvE nuker set.
Major Arcana also has another really useful function in PvP, it has a 50% stun resist. So if you don't have a full set of boss jewels Major Arcana is really useful against archers. Also, if you put PvP on your Major Arcana robe it lowers the chance of being critted by archers. For all other situations I use my DC set.
With the reuse-time of skills not being affected by castingspeed, and the new 300 attrib weapons in Gracia-final .. the only usable armor for casters 76+ will be the arcana robe.