simply put -
diesels product more power per gallon of fuel
diesels use less fuel per gallon/liter per unit time
diesels have fewer moving parts than gas powered
diesels produce more torque - and do not need to run high in the RPM range.
diesel typically last longer in hours than the gas counterparts
diesels have no spark
nor carburetor
diesels need to have the fuel very clean to run well.
diesels need a "different" oil in the crankcase than a typical gas engine.
As for maintenance - change fluids/filters and you'll be fine!
ONLY down side of a diesel is a cold weather start, and even that is not a problem!
IF you have any specific question - fire away.
They cost more in every way than a comparable gas engine (maintenance, repair, initial cost).
They dont' have fewer parts. Well some do. But what gas engine are we comparing to? 2 stroke diesel vs DOHC 4 stroke gas? Yeah big difference. A 30hp gas burner has one camshaft usually and a 30hp diesel will have 2. Most gas engines don't have a high pressure pump, mechanical injectors with moving parts, governors with several moving parts, etc. BUT...you're right....they make more TORQUE per unit of fuel than the same HP gas engine, and at a lower RPM. Horsepower, not so much. A similarly displacement gasoline engine will make less torque but "usually" a lot more horsepower because it can rev higher without self-destruction. It's not unheard of to see 300hp out of a 1.6L gas burner, but an L2501 has a 1.8L engine and makes 25hp; but it'll do it ALL day at full RPM and full load, and wants more. A 300hp gas burner ain't gonna do that. Well not usually.
Diesel engines typically last longer...the fuel is a lubricant of sorts and some unburned fuel in the cylinders lubricates the top end of the engine; thus is lasts longer. Also diesels are built stronger due to how the engine operates, thus the heavier parts combined with lower engine RPM tend to have a longer lifespan. Now with the newer DPF stuff, I don't know...they haven't been out long enough.
Diesel engines are heavier than most gas counterparts of the same horsepower. They have to be because of how the engine is used and how it is designed. The compression-ignition (no other source of ignition) places a tremendous load on internal parts and a gas burner's internal parts aren't subjected to the same loads and can be lighter weight. The weight of the diesel engine's parts also contribute to the lower engine speeds-a 5 lb piston doesn't like 6000 RPM for very long.
Diesel fuel has some problematic characteristics. It gels in cold temps. Not an issue in the southern USA. Algae grows in diesel fuel. Good to treat it with a fuel treatment. It also will go bad over time similar to gas; but in a diesel engine there is no carburetor to plug, so generally it just runs a little smoky and maybe a little low on power. Might run a little noisier as well depending on the engine. They're all different.
You don't want a gas burner tractor. Not at all. Ford made a bunch of them and once you got off a gas tractor and onto a diesel tractor, you'd want to drive the gas burner into a pond and forget about it. I've had both, currently have a MF1145, 1.5L diesel, turbocharged, 35hp. Not all diesels are turbocharged but the ones that are typically have more torque at a lower RPM, and that torque happens perfectly timed with loading. A turbocharger forces air into the engine. More air=better burn=more exhaust=spins the turbo faster=more air. It's a cycle and it flat works, and works excellent. IMO diesel engines were made to be turbocharged, even the old 2 cycles.
But because of the EPA's ruling on diesels, and the complexity of the systems to meet EPA exhaust emission standards, I keep wondering, is it about time to look at building a gasoline-powered tractor again? Hmm....with today's tech, a turbo small displacement gas engine could potentially make the same horsepower and torque as a diesel, but would use more fuel, and maintenance/repairs would potentially be a little cheaper.