Both sound good.. if you want to be safe, use V30, they seem to be allot easier to mic up.
But my favorite sounds have been recorded with G12's, i just think that they sound broader and crunchier, but at the same time, its hard to get a really good sound out of them, but when you do it definitely beats the V30(Thats my opinion.).
Edit:
Oh, and to make things clear: I dont play downtuned modern metal like most guys on this forum! ;P