Here are my 2 cents:
There are two main considerations when buying a generator...reliability and economy.
Since diesel engines are approximately 40% more economical than similarly rated petrol engines, besides being more durable and robustly built, a diesel powered generator is a better option. By necessity, diesel generators are sound proofed too.
So unless you can ensure uninterrupted natural gas supply, a diesel generator is a better option than a similarly rated petrol generator.
Now let me give you my example. Since 2008, I have been using a 5.0 kVA PEL diesel generator at my clinic. It's actually a rebranded Kipor KDE6700TA. I haven't faced any major issue with it and it keeps performing reliably running a load that comprises one 1.5 ton split AC, 1 PC, 1 laptop, 1 laser printer, 1 ultrasound machine, 1 exhaust fan, 1 bracket fan, 10 energy savers, 14 tubelights and a water dispenser. The parts are easily available locally. The same model is now marketed by Aurora (rebranded Aurora), Metro (rebranded Lister-Petter) and original Kipor (unauthorized imports) ranging in price from Rs. 95,000 - 140,000. Do remember Kipor is a world class Chinese brand that markets their lineup in Europe and North America.
Finally the ATS. Though I must admit I haven't installed one but if I was to get one, it'll have to be the Clipper ATS. Not only is it a smart piece of engineering, it's also a quarter of the price of similarly rated but bulkier and less advanced ATS systems. You can learn more about the Clipper ATS at https://www.facebook.com/ClipperATS.
Hope this helps.