I have the Kohler 14KW generator and it's running on NG that drops it down to 12KW. It was installed in mid 2012. Sure came in handy when that storm Sandy hit the east coast in Oct 2012.
I've heard pros and cons on the Kohler and Generac. Two of my neighbors have the Generacs.
We used Onans at work and had very good service from them. We used a lot of their gasoline sets.
When the military switched everything to diesel we bought diesel Onans.
We bought our generator in AZ and the set was shipped from TX. Hire a good plumber and electrician that has wired generators. The plumber used Wardflex piping in the basement and hard piped from the house to gen. Also have it inspected. Hal