Make sure there is enough gas pressure in your area to support a generator. Have your contractor put your data through his sizing program to find the correct kW rating you need.
Use a Service Entrance rated transfer switch so you don't accidentally back-feed your utility grid. Linemen have been killed when working on a line that is 'live' when it should be 'dead'.
Try to listen to various brands of generator in use to see if they are too loud for you.
Find out how much the maintenance plan costs and the service it provides.
Take a look at Kohler generators. They are well-built and made in the USA.