I mean both, If you can come up with the proper apex seal diesel is a great fuel.

 

Having a smaller higher speed generator enables you to build it lighter per the amount of horsepower produced. Horsepower is what you need in order to get a certain amount of work done. However the torque level associated with that horsepower will determine how strong the engine structure needs to be. If we had 2 engines and one was a diesel and the other was gasoline and both made 50 hp, unless they can do it at the same rpm they will not be the same size or weight. The reason being that lets say the diesel engine makes 50 hp at 1500 rpm while the gasoline engine makes 50 hp at 6000 rpm. The diesel will have to be the much heavier and stronger built engine and it has nothing to do with the fact that it is diesel but rather the torque trying to twist the engine apart. The diesel will make 175 ft lbs of torque while the gasoline engine will only make 44 ft lbs. Since the gasoline engine has far less torque trying to rip it apart, it can be made lighter and smaller. Keep in mind that the diesel would also have to be larger in displacement as it's the amount of air and fuel used per time that matters so this too would effect size and weight. If the gasoline engine made as much power as the diesel at the same 1500 rpm, it too would have to built much stronger and heavier.

A turbine is a very efficient device within a fairly narrow rpm range which also suits itself well to a generator application.