Petrol - it warms up quicker and won't cause you DPF or similar expensive problems. Also the larger the engine in a car that size, the less it has to work hard so better fuel economy as a general rule od thumb. Obviously if you go for a 5.0V8 supercharged then economy won't be as good.
Whatever you go for, make sure to give it one long high speed (motorway) run every so often to help keep things cleared out.