Both types of fuel have pros and cons and I think they both have types of vehicles that they are best suited for.
Gas (Petrol) Pros: More horsepower, better performance as far as just general acceleration and speed, engines that aren't usually as heavy since the blocks and heads can be made from lightweight aluminum alloys.
Cons: Lower torque figures, not as useful for carrying or hauling heavy weight (bogging down more easily), lower mileage.
Best places for gas engines: Small cars, midsize and full sized sedans, mid-sized and smaller SUVs a, Sports cars, supercars. Any vehicle that needs a little pep in it's step and won't be expected to tow or haul anything super heavy.
Diesel pros: Better mileage, more low end torque, hauls more weight without bogging down.
Cons: Diesel engine blocks are made of cast iron, they have to be, so they're very heavy.
The powerband is very short; say you have a gas engine; 4.0 liter I-6, with a smooth continuous powerband up to a red line around 6000-6500 rpm. If that same engine were built to run on diesel it would provide adequate torque at much lower RPMs then it would drop off pretty early, all the horsepower would come in one short useless little lump and that would be it, time for a gear change. The red line would be about half as high as the gas version.
Best uses for a diesel engine: Pickup trucks, mostly full size and heavy duty. Commercial Trucks and vans. Full size and oversize SUVs. Really anything that would be expected to tow or haul a lot of weight, and/or climb serious hills without bogging down too much, but wouldn't necessarily need to go fast.
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"Ok, systems check; the battery is discharging, the oil temperature is very high, the oil pressure is very low, the engine temperature is off the end of the scale, I'm running out of petrol... but the clock is correct!"
-James May, in an old, worn out Lamborghini purchased for less than £10,000.
