600 watts is overkill for guitar and dosnt mean it will sound good.Your audience isnt going to hear much of your amp.They are going to hear the P.A.The volume between 120/300 watts and 600 watts is negligible as well.

And is it coming out of 2x12's!?
It dosn't matter if you have even 10,000 watts pushing through 2x12's,your still not going to push anymore air than 4/8x12's.