I live in the Vancouver area and we are well overdue for a major earthquake. Often times when there is a major earthquake the power goes out. This is all well and good when you have that emergency generator that you aren't even aware of hiding somewhere in the basement behind a boiler. In the event of an earthquake the natural gas lines are usually shut off which means at least half the buildings in the downtown area will be 100% without power. Diesel generators will not have this problem and most of the larger buildings will be fine.
Load testing makes up about 50% of my work. I take a dummy load to apply full load to the generator and make sure it can handle it. The small gas generators are rated for propane or gasoline and using natural gas de-rates the generator. Natural gas engines have a power output of 30% less BTU per unit than that of propane or gasoline. You have to be careful when loading a "15 kw" generator that you do not overwork the engine. I have tried applying full rated load to several small natural gas generators and they bog down terribly. Why don't they label the full load rating for natural gas as well? If the question is "why?" the answer is always money. I'm sure from a salesperson's viewpoint it sounds less appealing to mention "oh by the way for your application this 15 kw generator is actually only good for up to 12 kw." For the most part these generators loads never exceed 60% of full rated load anyways but you definitely want to be aware.
Natural gas burns a lot cleaner than other fossil fuels. It still produces greenhouse gases, unlike propane, but it emits 30% less carbon dioxide than oil. If natural gas does leak it is lighter than air so it rises up rather than creating a potential bomb. I think the major pro for using natural gas for your generator is the convenience. You never need to worry about what the fuel level is at and how you are going to refuel it. You don't have to worry about the diesel getting old or doing a chemical test for water. Unlike a diesel engine you don't have to worry about priming the fuel system or changing fuel filters.
My main "pet peeve" with natural gas fired generators is that they are usually back in a dark corner of a boiler room that is over 30 degrees celsius and are right on the ground making it a pain to work on them but that's just me whining :)
Small Onan natural gas generator
The brains of an Onan generator. The switch inside is to manually start the generator rather than running from the transfer switch. If there is no switch you just pull off wire #2 on the terminal strip
The first time I have seen a Coleman generator installed at a business. It was a natural gas unit as well.
A small Kohler natural gas unit. It has a belt driven starter/DC alternator hybrid.