I subscribed to Petra's site for several years, and she was a big fave of mine for a long time. But I sitll have to say that just for sheer mass, Janet is bigger. But this is like arguing over whether Coke is better than Pepsi--both sweet, both refreshing, take your pick.
Oh, and by the way, as far as I know, both girls are 100% natural. The guy who said that Petra was augmented is demented.