Diesel or Hybrid - which is more efficient?

I was in a discussion with a friend about buying a diesel or a hybrid - which is more efficient?

Well, the new generation of Turbo Diesels achieve better miles per gallon than comparable gasoline engines. Take a look at the Volkswagen Jetta: the TDI gets 41mpg highway, versus the gasoline model that gets 30mpg highway.

But is miles per gallon a good metric? I don't think so, as it is ignoring the energy content of the fuel. As a mindgame, suppose somebody concentrated gasoline such that it had half the volume, but the same energy content. The gasoline car would double it miles per gallon, but clearly we can't say that it is more efficient.

I would define efficiency as to what extent the amount of useful energy output from the fuel has been maximized.

It turns out that diesel contains about 9% more energy than gasoline by volume, so that contributes some to the increased miles per gallon. The other contributor to increased efficiency is that the diesel cycle is inherently more efficient than the gasoline cycle, due to higher compression ratios, and higher temperatures. Check out energy contents of fuels.

So, the TDI is more efficient in the use of resources than the gasoline, but not by as much as the raw mpg numbers indicate.

The hybrid has the advantage of capturing and storing energy that would otherwise be wasted. Both gasoline and diesel engines are wasting energy while stopped at traffic lights, braking, stop and go traffic. The hybrid is storing this energy.

Perhaps a diesel hybrid is the way to go.