I just had this pleasant dilemma and went for the 5.0 as it ticked more boxes in terms of power and more modern interior. The 5.0 has really useful shove and sounds glorious at full chat. My main reasoning on the power was that my last car was 303bhp and I wanted to feel I had gone up in spec. 4.2 vs 5.0 is probably better decided by yourself from the drivers seat I'd say.