The comparison you should be making is with the W12 for economy, and both will kill it dead
Cruising at the hideously boring speed of 65mph got me an average of 45ish from Dundee to nearly Glasgow, including a few roundabouts and hills. I'm averaging 30ish with a good bit of town driving and a heavy right foot with a cruise of around 80-85.
When you mash the loud pedal, it just goes, and goes, and goes, and keeps going until you wimp out. If you can find a good one, get the 4.2 over the 3.0, you'll regret the reduction in power especially when you're used to the W12.