Make yourself familiar with therm "thermal efficiency".
That translate how much of the energy from burning fuel is converted into ponnies.
For best, big diesels it exceed 50%, while in cars it gets about 45%.
Gasoline engines oscilate around 33%.
So not only diesel give better efficiency, but also generate less heat while doing so.
That's why diesel are used in pushers with limited cooling where gassers would cooked up.
The pollution is political question. Since diesels and gasser emit different pollution you can play with judging what is worse.
Per new proposed European system -American hybrids are gross polluters.
Diesels offer several advantages over gasoline for efficiency and economy.
1) Diesel fuel has more BTU's gal/liter than gasoline, IIRC it is about 20% higher. this doesn't affect efficiency but does affect economy.
2) Diesels generally operate at much lower rpm than an equivalent gas engine, therefore have lower frictional losses. since frictional losses in rotating machinery is proportional to rpm squared, a small change (up or down) in rpm has a big effect on losses.
3) Diesels are unthrottled, therefore much lower pumping losses. a gasoline engine has to draw air in through a restriction (throttle plate) diesels generally don't have or need a throttle plate. (There are now some diesels with a throttle plate to increase frictional losses during cold start to shorten warm up time for emissions) this results in considerable friction losses at idle and light throttle operation
4) diesels operate at lower peak pressure and temperature therefore have lower heat loss, especially at light loads. Since they don't need a constant air/fuel ratio like a gasoline engine, they run extremely lean at part throttle, air/fuel ratios of 200:1 are not uncommon. Meanwhile a gas engine has to run near 14.7:1. Low fuel consumption means little air which means low throttle openings which means very high restriction which means high frictional losses- get the picture?. While a gas engine will often overheat at idle, most diesels will see a signficant DROP in coolant temperature after a long idle. They run extremely lean at idle, for example my 6.6l duramax diesel (350HP, 650lb/ft torque), uses 0.5 liters of fuel/hour at idle. My 2.6 liter honda accord gas engine uses almost 3x that at idle! I've seen graphs showing the theoretical efficiency of a compression combustion engine (diesel) vs. a gasoline spark ignition (gasoline) engine. I don't recall the exact numbers but as mentioned theoretical efficiency of a diesel can exceed 50%, the most efficient internal combustion engine in the world is a sultzer (sp) marine diesel. over 5 million lb/ft of torque at 60 rpm.!
from what I remember, getting even 25% peak efficiency out of a current car gasoline engine is VERY difficult, whereas a car or light truck diesel is easily in the high 30% range.
5) the diesel combustion process results in a continous burn and near constant pressure for up to 90 degrees of crank rotation (the full length of the stroke). This is why they have such extremely high torque at low RPM vs. a gasoline engine. A gasoline engine has a quick burn for only a short period of time then relies on gas expansion most of the stroke.
6) There is almost no limit to the amount of turbo boost you can apply to a diesel, which helps increase volumetric and thermal efficiency. 2x atmospheric pressure (25-30 lbs) of boost is common on diesels. On a gas engine, there is a limit to boost that you can use to increase efficiency since it increases compression, and leads to pre ignition. 5-10 lbs seems to be a pretty common upper limit on production automotive gas engines for turbo boost.
7) This I haven't verified, but my belief is that a gasoline engine also suffers considerably under heavy load vs. a diesel. While the engine may be at full throttle, therefore not having much in pumping losses, ignition timing must be retarded considerably under heavy load (ie passing, accelerating, going up a hill etc.) to reduce pre ignition. This reduces efficiency considerably. For those of us old enough to remember distributors with vaccuum advance, may remember under light throttle ignition timing is advanced by a considerable amount under light throttle to improve mileage. If the vaccuum advance diaphram broke and you only had mechanical advance, mileage with drop considerably even under light load.
Hope this helps.