I-IV is more active than either, actually.
From what I've observed I'd say the level of activity for the gaming forums goes:
General Final Fantasy
Final Fantasy X
Final Fantasy X-2
Final Fantasy VII
Final Fantasy VIII
Kingdom Hearts
Final Fantasy I-IV
Final Fantasy XI
Final Fantasy IX
Final Fantasy VI
General Square Enix
Chrono Cross / Chrono Trigger
Final Fantasy XII
Video Game Music / TSW Forum
Final Fantasy Tactics
Final Fantasy V
Final Fantasy Crystal Chronicles
Final Fantasy Tactics Advance
