Gasoline and natural/bottled-gas engines are "the same thing". You replace the liquid carb with a gaseous carb. There are differences in power: good natural gas will make more power in a given size than gasoline, if the engine is designed to run on good natural gas. But you can't get the good stuff outside the gas fields (street gas is diluted to a low common spec, bottle gas is just a little richer) and most engines under 200HP (130KW) are gasoline engines with hasty adaptations.
> A diesel genset is more efficient and more reliable.
A diesel will always cost more and weigh more than a spark-engine of the same power. Spark-ignition goes "fwoosh", Diesels go "BANG!". A Diesel always runs harder than a worst-case sparker, and has to be built about 3 times stronger to live at all. 3 times stronger also means it can't run as fast. However the resulting heavy expensive engine will run a looooooong time before wear is a problem. Diesels are slightly more efficient than sparkers at full load, but much more efficient at part load.
If you only run a few hours a year, the up-front cost of a Diesel will never pay-back in fuel or replacement costs.
If you run many hours a year, the Diesel is miles ahead on fuel and ring-job costs.
As a very rough rule: less than 1,000 hours in the next few years, get a cheap sparker. Cars run 2,000 hours in 10 years and, at US fuel prices, are mostly sparkers. Simple economics. Heavy business trucks run 2,000 hours in one year, and are mostly Diesels. Same economics.
In Bob's situation, up to 250 hours every year or two, the mass-market sparker is the "obvious" way to go. Especially since he can lose the ~$3,000 cost of a 13KW sparker genset in just a few days of lost business.
For Roger's friends who live off-grid, runtime/year is much higher and the balance shifts toward Diesel. If you run much of the day almost every day, Diesel will usually pay-back.
13KW of Diesel seems to run $10K, $15, and up and up.
http://NorthernTool.com has pictures and prices for a wide range of gensets, from a 1KW job with a handle (looks like a beercooler with an AC outlet) to a 150KW V10 that self-tests weekly for $22K. Northern isn't real good about having what you want in-stock, but they do catalog a wide range and give you low-ball prices.
Northern does list the RV motor-home modules. These seem to span 4KW to 7.5KW, which will run "two 13,500BTU A/Cs with 1,500 to 4,000 watts reserve". They are boxed so you can slide them under your RV floor and sleep.
Oh: you could build a heavy sparker to run as slow, and quieter, than a Diesel. Nobody does. If you are going to invest that much money and metal, Diesels ARE more reliable. The spark system on sparkers is usually cheap and gives much trouble. A Diesel has potential trouble in the injection system, but can't be cheap so is usually quite reliable. Sparkers had carbs which were often junk; this point fades because sparkers now use injection systems, and Diesels are adding (unreliable) computers to their injection (for smog and knock control).
You want slow: I know a spark engine that is red-lined at 750 RPM. I understand it is not quiet. It is as big as a house, and pumps a lot of natural gas through the very long skinny pipe from Tulsa to NYC.
The difference in cost between a big slow engine and a small fast engine will buy a lot of sound baffling. That seems to be the trend. And in your situation, $1K of feeder cable will buy a lot of sound loss through distance away from the mastering and sleeping suites (assuming you have the land).
There is another trend. Traditionally an alternator HAD to run 600, 1,200, 1,800, or 3,600 RPM to stay on 60Hz. 3,600RPM is an excellent speed for a 20HP spark engine. But that means when all the A/C kicks out and the lights are dimmed, the engine is still roaring along at freeway speed. However, new/smarter power converters make it possible to slow the engine at low load, yet still stay locked on 60Hz. This could be a big difference when you don't need your entire rated power. My Honda sedan cruises much quieter in 5th at 2,500RPM than it does in 4th at 3,500RPM. Power is 2/3rd but noise may be half, even flat-out.
Ya know, that's an interesting comparison. You already have a lot of 13KW engines going past the house: cars on the road cruise around 20HP. On many cars, the tires make as much noise as the engine. If you can tolerate that, it should be possible to tolerate an enclosed engine, stationary (no Doppler), half as far away as the road.
You could just move. Here outside NYC, on a former rural electric supply, I have not lost power more than 4 hours in the last decade. Knock wood.... last year the town north of me was out for 3 days, and the podunk electric company was in deep trouble. They'd run lines through trees for years, hadn't kept up the pruning, or the redundant routing, and one freak windstorm overwhelmed their repair crews.