Either way Squaresoft/Enix is going to charge you $13.00 for the monthly fees. The PS2 is going to cost a little more becuase you'll need the new HDD (Hard Disc Drive) upgrade, Network Adapter, plus the game. Ouch.
But....it looks pretty interesting for a MMORPG. I also think this is the first online gaming venture for Squaresoft/Enix (please correct me if I'm wrong).