Look at BTU's for diesel, gas, biodiesel, and other alternatives.
Look at emissions and the type of emissions.
Look at the emissions when petro diesel is combined with biodiesel.
Also keep in mind that the entire world could not (will not) go 100% diesel or biodiesel. If everyone went diesel, it would drive the diesel prices sky high, and make gasoline really cheap.
You may also want to do some research on fuel-to-energy efficiency, which would include diesel-electric motors. To me, the real future is going to be electric. Diesel-electric is one step closer (trains and ships). Nuclear-electric (Navy ships and subs) is another step. Fuel cell cars is another step.
Or read up on the Picken's plan regarding using natural gas as another cheap and viable alternative.