For hauling heavy stuff off road I would have to say the PSD. Maybe the V10 if you're not concerned with fuel economy... My boss has a 7.3PSD Excursion with 4" exhaust from the turbo and a supertuner program set to 80hp and this truck is awesome... No problem pulling 9000lbs. He gets about 18-20 mpg empty and about 10-13 loaded... I wonder what the 6.0 would do with the same mods... He also has a Banks trans-command which acts like a shift kit...