Good Question. I would think Gas is better due to lighter engine block. Desiel typical is high torque low HP and better milage. I know in the next few years some high end sport cars are looking to switch from v12 gas to 10 cyclinder desiel motors. The tatic is to gain better miliage keep polution output same or lower by doing so. They are installing different transmission and drive train to accomedate the high togue of the desiel motor and covert that into rpm on the drive wheels.
The manufactures are very concerned about handling issues of the heavy desiel motor block.
Given the changes seen in F1 racing I think they might be right! F1 cars are switching back to the smaller motors adn getting better performance from the cars. Sure the v12 is faster but you slow down too much in the bends and acceration is no better than the v10.