The Arsenal will handle a lot more than 600W RMS if you ever got to upgrade. It will be louder, but I bet the Diamond has slightly better SQ, but the Arsenal will probably be "flatter".
That's a tough comparison, but the Arsenal will probably overall be better...
Casey, I don't think enough peolpe have the Arsenals yet. They sold out in Feb. and have not sent any out since. I'm still waiting on my 12s. I have asked many times how they sound without much response. I'll just have to keep waiting to hear for myself.