Natural Gas seems like the best fuel because it shouldn't run out. I understand that something can happen to the pipeline, but there are more chances that you won't be able to get gas, diesel or propane then the Natural Gas Line not supplying it.
MY current generator is a 10,000 watt starting 8,000 watt running Champion generator that also used propane, but the rating decreases to 9,000 watt starting and 7,200 watt running rating. I have several propane bottle for it because it can be challenging getting gasoline here when we have a Hurricane/Tropical Storm, or even worse, an ice storm that closes the roads. When the power is out, most of the gas stations shut down because they do not have generators. The last outage, I had four 5 gallon gas cans filled up and I used 3 of them. A couple years ago, we lost power for a week because of an ice storm, and I used up all my gasoline and didn't have any propane, so I had to drive around to find an open gas station. I think I found gasoline somewhere around my 20th gas station.
For me, if I'm going to spend the money on a whole house system, I want it to run off of Natural Gas.