Natural gas is the way to go as long as you are not in an earthquake prone area.
I run a 5500 watt portable because we don't have NG. I looked into the propane units but they burn way more than gasoline especially in the dollar per hour aspect. I typically burn about 8-10 gallons per 24 hours. Thats when I'm at home because work is closed. A little less once we stop using stuff around the house.
I run 2 fridges, 1 1/2hp sump pump, and blower on the gas hot air furnace.
I installed a interlock device in my main panel, so every circuit has power in the house if I want it. I can walk around use any light and any outlet.
Now granted I know I can't use the electric range or dryer. But I can use the microwave, coffee maker, and stuff like that.
I have carefully run 1 burner on the electric stove and done a load of dishes in the dishwasher. Not at the same time and with almost everything else off.
When we have lost power during the hurricane force rain storms the sump pump is fine, after the rain is passed I unplug the regular sump pump and let the battery unit take care of the water if need be. The battery charger does not use much power and has no start surge like the 1/2hp regular pump.
You have to look more realistically into what you want to power. Big things in the equation are the stove and AC.
Depending on the size of the a/c is going to be the biggest hog.
You don't necessarily need to be able to power both. Say you want to cook, turn of the a/c for a bit.
Or calculate the range on maybe on using the oven or 2 burners max.
The 2 most common brands of bigger generators you will see are Generac and Kohler. Kohler makes a better unit from what I have heard. Most of the bigger ones will use a GMC engine in them. You should look into 20-30 kw range. Expect to spend at least 5-10k on a unit.
20kw
http://www.northerntool.com/shop/tools/product_200448671_20044867130 kw
http://www.northerntool.com/shop/tools/product_200481398_200481398If you want to look at other brands, Onan, and Caterpillar are great also. But big bucks